Systems and methods for accurate, stable voltage supply

ABSTRACT

A voltage supply and a method for calibrating the voltage supply are provided. The voltage supply is for providing a reference voltage to supply a voltage to at least one electrode. The voltage supply comprises: an ultra-stable DC voltage source, an accurate DC voltage source, a tuning unit, a comparator, and a control unit. An ultra-stable voltage is applied to the tuning unit, which is provided based on a supplied voltage of the ultra-stable DC voltage source. The tuning unit provides an output voltage. A voltage based on the output voltage of the tuning unit is compared by the comparator with an accurate voltage. The accurate voltage is provided based on a supplied voltage of the accurate DC voltage source. The comparator provides a signal resulting from the comparison to the control unit, wherein the control unit is tuning the tuning unit during a tuning period according to the signal provided by the comparator to minimize the absolute difference between the voltage based on the output voltage of the tuning unit and the accurate voltage. The reference voltage of the voltage supply is provided based on the output voltage of the tuning unit after the tuning period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority to GB Patent Application No.2001107.8, filed on Jan. 27, 2020, which is hereby incorporated hereinby reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a voltage supply, which is providing areference voltage. This reference voltage is used to supply a voltage toat least one electrode.

BACKGROUND OF THE INVENTION

In a mass spectrometer, ions are transferred and analysed by chargedelectrodes. DC voltages and/or RF voltages are applied to theseelectrodes.

The performance of the mass spectrometers depends very often on thestability and/or accuracy of the applied voltages. This can be providedby a voltage source, a voltage supply or a reference voltage which isprovided by a voltage supply and which can then be amplified by avoltage amplifier to the voltage which is applied at an electrode.

The requirements regarding the stability and accuracy of the appliedvoltages depend on the function of an electrode in a mass spectrometer.

A voltage provided by a DC voltage source for some functions in a massspectrometer, should be stable over time and temperature.

Therefore the stability and accordingly precision of a voltage providedby a DC voltage source is defined over time and temperature and haspreferably the same value regarding a specific time period andtemperature range.

The stability of a voltage of a DC voltage source over time is definedas 6 times the standard deviation of the voltage in a specific timeperiod at a constant temperature (ΔT<1° C.). Typically the specific timeperiod to define the stability of a voltage is between 12 and 36 hours,preferably it is defined for a time period of 24 hours. The stability ofthe voltage over the time is mostly provided as the 6 times the standarddeviation of the voltage in relation to the average value of the voltagesupplied by the DC voltage source and is typically provided as a ppmvalue. So when the average voltage of a voltage source is 6 V and thestability of the voltage over the time is 12 ppm, the standard deviationof the voltage is 12 μV in the specific time period defining thestability of the voltage source.

The stability of a voltage of a DC voltage source, which is the averagevoltage provided by the DC voltage source, over the temperature isdefined as the maximum deviation of a maximum value and a minimum valueof the voltage in a specific temperature range. Typically the stabilityof a voltage is defined over a temperature range of at least 10° C.,preferably of at least 20° C. and particular preferably of at least 25°C. Preferably the temperature range is symmetrical regarding anoperation temperature of the DC voltage source.

This stability of the voltage of a DC voltage source over thetemperature can be available at a specific operation temperature of theDC voltage source. Preferably it is available at a range of theoperation temperature of the DC voltage source. The range of theoperation temperature, at which the stability of the voltage of a DCvoltage source over the temperature is available, can be larger than 2°C., preferably larger than 10° C., more preferably larger than 50° C.and most preferably larger than 100° C. A typical range of the operationtemperature, at which the stability of the voltage of a DC voltagesource over the temperature is available, is between 15° C. and 40° C.Preferably the range of the operation temperature, at which thestability of the voltage of a DC voltage source over the temperature isavailable, is between 5° C. and 60° C., more preferably between −10° C.and 90° C. and most preferably between −40° C. and 110° C.

The stability of the voltage over the temperature is mostly provided asthe relative maximum deviation of the maximum value and the minimumvalue of the voltage in the specific temperature range in relation tothe average value of the voltage supplied by the DC voltage source andis typically provided as a ppm value. So when the average voltage of avoltage source is 6 V and the stability of the voltage over thetemperature is 10 ppm, the maximum deviation of the voltage is 60 μV,which is the maximum difference of the voltage in the specifictemperature range, for which the stability of the voltage source isdetermined.

The accuracy of the DC voltage supplied by a DC voltage source isdefined by the maximum deviation, which the average value of thesupplied voltage has, when the DC voltage source is produced. Thisaccuracy is further denoted as production accuracy. So the average valueof the supplied voltage of a DC voltage source does not deviate morethan the maximum deviation, given by the accuracy, from its nominalvoltage, which is the value of the voltage, which shall be provided bythe DC source.

The accuracy of the DC voltage supplied by a DC voltage source isprovided as a ppm value, providing the ratio of maximum deviation of theaverage value of a produced DC voltage source from its nominal voltageto its nominal voltage. Typically the deviation of the average values ofthe supplied voltage of a set of produced DC voltage sources issymmetrical to their nominal voltage. But it is also possible that thedeviation to higher or lower voltage values regarding the nominalvoltage has a higher occurrence e.g. due to a systematic error.

In particular, the performance of a time-of-flight (TOF) massspectrometer depends on the stability and or accuracy of the voltages,applied at its electrodes.

In a time-of-flight (TOF) mass spectrometer the flight times of ionstravelling a known distance are recorded. These flight times are used todetermine the mass to charge ratios (m/z). The translation from theflight time to the mass to charge ratio is done by a calibrationfunction.

In multi-reflection time-of-flight mass spectrometers ions areoscillating between ion mirrors of the mass analyser, which comprise atleast one mirror electrode, the stability of the calibration of thesetime-of-flight mass spectrometers is influenced, among other parameters,by the stability and accuracy of the voltages applied to the mirrorelectrodes. The ratio of the mass accuracy of the mass spectrometer tothe voltage stability of its mirror electrodes, to which a stablevoltage is supplied, is more or less one to one. Therefore, e.g. toachieve a typically intended mass accuracy of 1 ppm, a voltage stabilityof 1 ppm is required for at least one of the mirror electrodes. For afurther increased high resolving power and mass accuracy of the massspectrometer a voltage stability below 0.8 ppm of at least one mirrorelectrode is advantageous and a voltage stability below 0.5 ppm of atleast one mirror electrode is more advantageous.

Also the performance of a Fourier transform mass spectrometer, which cancomprise an electrostatic trap as mass analyser, depends on thestability and or accuracy of the voltages, applied at its electrodes.

In a Fourier transform mass spectrometer the ions are cycling in an iontrap. The measured cycling frequency of the ions is used to determinetheir mass to charge ratios. The translation from the frequency to themass to charge ratio is done using a calibration function. The stabilityof the calibration is influenced, among other parameters, by thestability of the voltages applied to the electrodes of the ion trap, inparticular to the center electrode of an electrostatic trap, e.g. thecenter electrode of an Orbitrap® mass analyser distributed by ThermoFisher Scientific Inc. The ratio between the mass accuracy of the massspectrometer and the voltage stability is more or less one to one.Therefore, to achieve a mass stability of 1 ppm, a voltage stability of1 ppm is required.

There are very few accurate voltage references available which claim atemperature stability of 1 ppm/° C. But this is not sufficient tooperate a mass spectrometer within a common temperature range of about10° C.

In particular in high precision voltage instruments voltage supplies areused, which provide reference voltages which are extremely stable andprecise but unfortunately not accurate. The output voltage of theseultra-stable DC voltage sources can be in the range of 2 V to 20 V,mostly in the range of 5V to 10 V and preferably in the range of 5.5 Vand 8 V. But the output voltage typically differs considerably,typically by more than +/−4%. These ultra-stable DC voltage sources arein particular using Zener diodes. Examples of such voltage supplies arethe reference voltage sources LM399 and LTZ1000 distributed by LinearTechnology Corp.

The accuracy needed in mass spectrometry for a reference voltage is notas high as the needed stability. Nevertheless, +/−4% tolerance is inmost cases not accurate enough to successfully operate a massspectrometer.

In many applications, for example in digital multimeters, a stablereference voltage is used and the accurate reading value of a DC voltageis tuned with a calibrator. This can be done by potentiometers to adjustthe gain in the reference circuit to produce the required accuratevoltage value. In most modern test and measurement instruments that arebased on a stable reference voltage the analogue to digital converterrange is large enough to deal with reference voltages over the wholetolerance band. The calibration is done in software using this approach.In both approaches, calibration for every single instrument is requiredin order to meet the specifications. These production methods are timeconsuming and expensive for several reasons. Precise and costlyequipment is indispensable and in addition trained personal is neededwhich can carry out an accurate calibration. Other disadvantages are therelatively bad stability of potentiometers, which disturb the stabilityof the circuit. The need of a wide tuning range is also a source ofinstabilities in high voltage applications. The smaller the tuningrange, the smaller is the produced influence of the circuit.

In summary, using a lot of effort is required to transform anultra-stable and precise voltage references into an accurate voltagereference with the same stability. Furthermore, the techniques to get anaccurate reference voltage out of a precise voltage will influence itsstability.

It is therefore the first object of the invention to provide an improvedvoltage supply, which is providing a reference voltage, which has a highaccuracy and also a high stability.

It is a second object of the invention is to provide a calibrationmethod for the improved voltage supply, which is providing a referencevoltage, which has a high accuracy and also a high stability, whichmakes it possible that the reference voltage is provided by the voltagesupply as soon as possible after the voltage supply has been activated,in particular reactivated after an unintended disruption of the voltagesupply, e.g. by power breakdown.

It is a third object of the invention that the provided calibrationmethod is already providing a reference voltage, before the calibrationof the reference voltage having a high accuracy and high stability isfinished.

SUMMARY OF THE INVENTION

The first object is solved by the voltage supply of claim 1.

The voltage supply provides a reference voltage to supply a voltage toat least one electrode. The provided reference voltage can be applieddirectly to the at least one electrode or can be applied to at least oneamplifier, which then is providing an amplified voltage which issupplied to at least one of the electrodes. The supplied referencevoltage can be used to supply a voltage to more than one electrode. Thesupplied voltage provided by the at least one amplifier as amplifiedvoltage can be the same for all electrodes or different for someelectrodes or each of the electrodes.

The inventive voltage supply comprises two different DC voltage sources,an ultra-stable DC voltage source and an accurate DC voltage source.

The ultra-stable DC voltage source provides a very stable outputvoltage. The stability of the output voltage is provided over the timeand a specific temperature range. Typically the output voltage provideby a DC ultra-stable voltage source has a voltage stability below 5 ppm,preferably below 1 ppm, more preferably below 0.5 ppm and particularpreferably below 0.3 ppm. The stability of the output voltage isprovided typically over more than 12 hours, preferably over more than 24hours, more preferably over more than 48 hours and particularly overmore than 96 hours. The stability of the output voltage is providedtypically over a temperature range of more than 10° C., preferably overmore than 15° C., more preferably over more than 20° C. and particularlyover more than 25° C.

The stability of the output voltage of the ultra-stable DC voltagesource over the temperature range can be available at a specificoperation temperature of the ultra-stable DC voltage source orpreferably at a range of the operation temperature of the ultra-stableDC voltage source.

The range of the operation temperature, at which the stability of thevoltage of the ultra-stable DC voltage source over the temperature isavailable, can be larger than 2° C., preferably larger than 10° C., morepreferably larger than 50° C. and most preferably larger than 100° C. Atypical range of the operation temperature, at which the stability ofthe voltage of the ultra-stable DC voltage source over the temperatureis available, is between 15° C. and 40° C. Preferably the range of theoperation temperature, at which the stability of the voltage of theultra-stable DC voltage source over the temperature is available, isbetween 5° C. and 60° C., more preferably between −10° C. and 90° C. andmost preferably between −40° C. and 110° C.

The accurate DC voltage source provides a very accurate DC outputvoltage with an accuracy typically below 1000 ppm, preferably below 400ppm, more preferably below 250 ppm and most preferably below 100 ppm.

The voltage supplied by the ultra-stable DC source can have preferably ahigher absolute value than the voltage supplied by the accurate DCvoltage source. In this case additional amplifiers are not required inthe inventive voltage supply.

Typically in these embodiments the absolute value of the voltage of theultra-stable DC source is at least 2% higher than the absolute value ofthe voltage of the accurate DC voltage source. Preferably the absolutevalue of the voltage of the ultra-stable DC source is at least 10%higher than the absolute value of the voltage of the accurate DC voltagesource. More preferably the absolute value of the voltage of theultra-stable DC source is at least 25% higher than the absolute value ofthe voltage of the accurate DC voltage source.

Typically the absolute value of the voltage of the ultra-stable DCsource is not more than 500% higher than the absolute value of thevoltage of the accurate DC voltage source. Preferably the absolute valueof the voltage of the ultra-stable DC source is not more than 200%higher than the absolute value of the voltage of the accurate DC voltagesource. More preferably the absolute value of the voltage of theultra-stable DC source is not more than 100% higher than the absolutevalue of the voltage of the accurate DC voltage source.

In another embodiment the voltage provided by the ultra-stable DC sourceis supplied to an amplifier and then the amplified voltage has a higherabsolute value than the voltage provided by the accurate DC voltagesource.

Typically both voltages sources, the ultra-stable DC source and theaccurate DC voltage source provide voltages with absolute values in therange of 0.5 V to 100 V, preferably in the range of 2 V to 20 V and morepreferably in the range of 5 V to 10 V and most preferably in the rangeof 5.5 V to 8 V. Preferably ultra-stable DC voltage sources are usingZ-diodes and/or Zener diodes, in particular based in silicon, whichpreferably are supplying voltages in the range of 5 V to 8 V.

The inventive voltage supply also comprises a tuning unit.

A voltage is supplied by a voltage source to the tuning unit, which isapplied at at least one (input) connector of the tuning unit, preferablytwo (input) connectors of the tuning unit. The tuning unit provides anoutput voltage to at least one (output) connector, wherein the providedoutput voltage can be adjusted by the tuning unit.

The tuning unit can comprise one or more tunable voltage dividers. In apreferred embodiment, the tuning unit can be a tunable voltage divider.

A tunable voltage divider comprises preferably at least one resistor,more preferably a network of connected resistors. Then the tunablevoltage divider provides an output voltage to at least one (output)connector by tapping the voltage from the resistor or the resistornetwork.

A tunable voltage divider which can be used in the inventive voltagesupply is a voltage divider, which is tunable, which means that theoutput voltage can be adjusted. A kind of tunable voltage divider isalso named potentiometer. In a classical potentiometer the voltagedivider is tuned by a sliding contact, which is connected to an outputconnector. A tunable voltage divider of the inventive voltage supply canbe tuned analogue or digitally. In an analogue tunable voltage dividerthe tuning can be executed also by an electrical component of theelectrical circuit of the voltage divider, due to which a tuned outputvoltage is provided to an (output) connector of the voltage divider. Ifthe voltage divider is digitally tuned, it can be a digital to analogueconverter (DAC), to which a digital signal of several bits is providedas tuning input, wherein the bits are switching resistors in a parallelresistor network of the digital to analogue converter (DAC), which can aR-2R ladder DAC comprising a repeated cascaded structure of resistorshaving the values R and 2R. In this case each bit is binary weighted.

Another type of digital to analogue converter (DAC), which can be usedas tuning unit in the inventive voltage supply, comprises a pulse widthmodulator, which is controlled by a single bit signal, and a low-passfilter to provide the DC component of the modulated signal as the outputvoltage of the tuning unit.

In another embodiment of the invention the tuning unit of the voltagesupply comprises at least one resistor and a tunable voltage divider,which are connected in series. When a voltage is supplied by a voltagesource to the tuning unit, only a portion of the voltage is applied atthe tunable voltage divider and only this portion of the applied voltageis tunable by the tunable voltage divider to provide an output voltageto the (output) connector.

In another embodiment of the invention the tuning unit of the voltagesupply comprises two tunable voltage dividers. The voltage supplied by avoltage source to the tuning unit is applied at a first tunable voltagedivider of the two tunable voltage dividers, which provides a firstoutput voltage to an (output) connector. This first output voltage isthen applied at a second tunable voltage divider of the two tunablevoltage dividers, which provides a second output voltage to an (output)connector, which is the output voltage of the tuning unit. Preferablythe first tunable voltage divider is used to tune the output voltage ofthe tuning unit coarsely and the second tunable voltage divider is usedto tune the output voltage of the tuning unit in a fine manner.

In another embodiment of the invention the tuning unit of the voltagesupply comprises at least one resistor and a digital to analogueconverter (DAC), which are connected in parallel and a current tovoltage converter, typically a transimpedance amplifier. Preferably thedigital to analogue converter (DAC) is connected in series with at leastone further resistor. When a voltage is supplied by a voltage source tothe tuning unit, this is applied at the parallel connected digital toanalogue converter (DAC) and at least one resistor. The other end ofthis parallel connection is connected with the input of the current tovoltage converter, which provides the output voltage of the tuning unit.

In another embodiment of the invention the tuning unit of the voltagesupply comprises at least one resistor and two digital to analogueconverters (DAC), which are connected in parallel and a current tovoltage converter, typically a transimpedance amplifier. When a voltageis supplied by a voltage source to the tuning unit, this is applied atthe parallel connected digital to analogue converters (DAC) and at leaston resistor. The other end of this parallel connection is connected withthe input of the current to voltage converter, which provides the outputvoltage of the tuning unit. In this configuration preferably one of twodigital to analogue converters (DAC) is provided for a coarse tuning ofthe output voltage of the tuning unit and the other of two digital toanalogue converters (DAC) is provided for a fine tuning of the outputvoltage of the tuning unit. Preferably both digital to analogueconverters (DAC) are connected in series with at least one furtherresistor. The digital to analogue converters (DAC) which is connected inseries with the resistor of higher resistivity is provided for the finetuning.

In another embodiment of the invention the tuning unit of the voltagesupply comprises at least one resistor and a digital to analogueconverter (DAC), which are supplied with two voltages, which aresupplied by two voltage sources to the tuning unit, and a current tovoltage converter, typically a transimpedance amplifier. The at leastone resistor and a digital to analogue converter (DAC) are connected ata node, which is connected with the input of the current to voltageconverter, which provides the output voltage of the tuning unit. Thedigital to analogue converter (DAC) is connected in series with at leastone further resistor.

In one embodiment of the inventive voltage supply the supplied voltageof the ultra-stable DC voltage source is applied directly at at leastone (input) connector of the tuning unit.

In another embodiment the voltage provided by the ultra-stable DCvoltage source is supplied to an amplifier and then the amplifiedvoltage is applied at at least one (input) connector of the tuning unit.

These embodiments represent simple arrangements showing how anultra-stable voltage based on the supplied voltage of the ultra-stableDC voltage source can be applied at the tuning unit. For example otherarrangements known by a skilled person might be used to apply a voltagederived from the supplied voltage of the ultra-stable DC voltage sourceas ultra-stable voltage to the tuning unit.

In one embodiment of the inventive power supply the output voltage ofthe tuning unit is used to supply the reference voltage provided by thevoltage supply. In this embodiment the tuning unit is providing thereference voltage directly as its output voltage.

In another embodiment of the inventive power supply the output voltageof the tuning unit is supplied to an amplifier and then the amplifiedvoltage is the reference voltage provided by the voltage supply.

In the whole specification the amplification of a voltage by anamplifier shall not be limited to the increase of the voltage which isamplified. Due to the amplification also a reduction of a voltage ispossible. According any amplifier described is in this specification canhave in general a gain which is typically above, but can be also below 1if a voltage is decreased by an amplifier.

These embodiments represent simple arrangements showing how thereference voltage of the inventive voltage supply can be provided basedon the output voltage of the tuning unit. For example other arrangementsknown by a skilled person might be used to provide the reference voltageof the inventive voltage supply, which is derived from the outputvoltage of the tuning unit.

The inventive voltage supply also comprises a comparator. A comparatoris an electrical component which compares two input voltages. To enabletuning of the voltage supply, the comparator provides an output signalwhich is resulting from the comparison of the two input voltages. Theoutput signal can be a signal which is equal or proportional to thedifference of the compared voltages. Preferably the comparator comprisesan operational amplifier which can be a differential amplifier.

In a preferred embodiment the output signal of the comparator is adigital signal which has only the output signal 0 and 1. By thesesignals it is only indicated which of the two input voltages has thehigher value. For such a kind of comparator can be used a high-gaindifferential amplifier, which is providing a very low or high voltage,which are then identified as signal 0 and 1.

The comparator can be also a digital comparator. By this comparator twodigital signals are compared. It is also possible to supply two analoguevoltages to a digital comparator, when they have been converted by ananalogue to digital converter. In this embodiment the digital comparatorpreferably uses a switch to measure both signals over the sameconnection with the same analogue to digital converter.

A voltage based on the output voltage of the tuning unit and an accuratevoltage, which is provided based on the accurate DC voltage source areprovided to the comparator of the inventive power supply, which comparesthe voltage signals.

In a preferred embodiment of the inventive voltage supply the outputvoltage of the tuning unit and the voltage of the accurate voltagesource are provided to the comparator of the inventive power supplydirectly, which compares the voltage signals.

In an embodiment of the inventive voltage supply the output voltage ofthe tuning unit is compared by the comparator with the accurate voltageto provide the signal to the control unit according to which the controlunit is tuning the tuning unit during the tuning period. By the tuningof the tuning unit the absolute difference between the output voltage ofthe tuning unit and the accurate voltage is minimised.

In another embodiment of the inventive voltage supply the voltage basedon the output voltage of the tuning unit, which is provided to thecomparator, is an amplified voltage of the output voltage of the tuningunit. Then the output voltage of the tuning unit is supplied to anamplifier, which is providing the amplified voltage of the outputvoltage. This amplified voltage is then used for the comparison by thecomparator.

These embodiments represent simple arrangements showing how to provide avoltage based on the output voltage of the tuning unit to the comparatorto be compared with an accurate voltage. For example other arrangementsknown by a skilled person might be used to provide a voltage, which isderived from the output voltage of the tuning unit, to the comparator tobe compared with an accurate voltage.

In an embodiment of the inventive voltage supply the supplied voltage ofthe accurate DC voltage source is the accurate voltage, which iscompared by the comparator with the voltage based on the output voltageof the tuning unit.

In another embodiment of the inventive voltage supply the accuratevoltage, which is provided based on the accurate DC voltage source andis provided to the comparator is an amplified voltage of the accuratevoltage supplied by the accurate DC voltage source. The accurate voltagesupplied by the accurate DC voltage source is supplied to an amplifier,which is providing the amplified voltage of the accurate voltagesupplied by the accurate DC voltage source. This amplified voltage isthen used for the comparison by the comparator.

These embodiments represent simple arrangements showing how to providean accurate voltage, which is provided based on the supplied voltage ofaccurate DC voltage source, to the comparator to be compared with avoltage based on the output voltage of the tuning unit. For exampleother arrangements known by a skilled person might be used to provide anaccurate voltage, which is derived from the supplied voltage of accurateDC voltage source, to the comparator to be compared with a voltage basedon the output voltage of the tuning unit.

The output signal which is resulting from the comparison of these twoinput voltages of the comparator, can be a signal which is equal orproportional to the difference of the compared voltages. The outputsignal which is resulting from the comparison of these two inputvoltages can be also a digital signal.

The inventive power supply comprises also a control unit.

The control unit may comprise a processor. A computer program may beexecuted on the processor to operate the voltage supply according to thedescribed steps of the method.

The signal provided by the comparator, which results from the comparisonof the voltage based on the output voltage of the tuning unit and theaccurate voltage, which is provided based on the supplied voltage of theaccurate DC voltage source is provided to the control unit as inputsignal.

During a tuning period, which is typically initially after activation orreactivation of the voltage supply, the control unit tunes the tuningunit according to the provided signal to minimise the difference betweenvoltages provided to the comparator which is the voltage based on theoutput voltage of the tuning unit and the accurate voltage, which isprovided based on the supplied voltage of the accurate DC voltagesource.

To achieve this, the control unit provides an output signal, which isprovided to the tuning unit.

If the signal provided by the comparator which is provided to thecontrol unit is equal or proportional to the difference of the comparedvoltages, according to the detected voltage difference the control unitreacts and provides a signal to the tuning unit to increase or decreaseits output voltage according to the detected voltage difference toreduce the difference of the voltages provided to the comparator.

The output signal provided by the control unit is increasing theabsolute value of the output voltage of the tuning unit if the absolutevalue of the accurate voltage, which is provided to the comparator basedon the supplied voltage of the accurate DC voltage source is higher thanthe absolute value of the voltage provided to the comparator, which isbased on the output voltage of the tuning unit.

When in the preferred embodiment of the inventive voltage supply theoutput voltage of the tuning unit and the voltage supplied by theaccurate DC voltage source are provided to the comparator of theinventive power supply directly, the output signal provided by thecontrol unit is increasing the absolute value of the output voltage ofthe tuning unit if the absolute value of the voltage of the accurate DCvoltage source is higher than the absolute value of the output voltageof the tuning unit provided to the comparator.

In particular if the comparator is providing a digital signal, then inan embodiment the digital signal provided by the comparator indicates bya first value that the absolute value of the accurate voltage, which isprovided to the comparator based on the supplied voltage of the accurateDC voltage source is higher than the absolute value of the voltageprovided to the comparator, which is based on the output voltage of thetuning unit. The control unit reacts on the first value of the digitalsignal provided by the comparator and provides a signal, preferably adigital signal, to the tuning unit to increase the absolute value of itsoutput voltage.

In this embodiment the output signal provided by the control unit isdecreasing the absolute value of the output voltage of the tuning unitif the absolute value of accurate voltage, which is provided to thecomparator based on the supplied voltage of the accurate DC voltagesource is lower than the absolute value of the voltage provided to thecomparator, which is based on the output voltage of the tuning unit.

The digital signal provided by the comparator indicates by a secondvalue that the absolute value of the accurate voltage, which is providedto the comparator based on the supplied voltage of the accurate DCvoltage source is lower than the absolute value of the voltage providedto the comparator, which is based on the output voltage of the tuningunit. The control unit reacts on the second value of the digital signalprovided by the comparator and provides a signal, preferably a digitalsignal, to the tuning unit to decrease the absolute value of its outputvoltage.

Preferably the increase and decrease of the output voltage of the tuningunit is reduced stepwise by the control unit providing accordingly asignal to the tuning unit. By this reduced change of the output voltageof the tuning unit the absolute difference between the voltages providedto the comparator, which is based on the output voltage of the tuningunit and the accurate voltage, which is provided to the comparator basedon the supplied voltage of the accurate DC voltage source is minimised,i.e. the absolute difference between the voltages provided to thecomparator, which is based on the output voltage of the tuning unit andthe accurate voltage, which is provided to the comparator based on thesupplied voltage of the accurate DC voltage source becomes progressivelyless.

For this approach it is only important, that the stepwise reduction ofthe change of the output voltage during the tuning process results in aminimisation of the absolute difference of the voltages compared by thecomparator.

When in the preferred embodiment of the inventive voltage supply theoutput voltage of the tuning unit and the voltage of the accuratevoltage source are provided to the comparator of the inventive powersupply directly, by the reduced change of the output voltage of thetuning unit the absolute difference between the output voltage of thetuning unit and the voltage of the accurate DC voltage source isminimised, i.e. the absolute difference between the output voltage ofthe tuning unit and the voltage of the accurate DC voltage sourcebecomes progressively less.

When in an embodiment of the inventive power supply the signal providedfrom the comparator indicates, that the absolute difference of thevoltage provided to the comparator, which is based on the output voltageof the tuning unit to the accurate voltage, which is provided to thecomparator based on the supplied voltage of the accurate DC voltagesource is below a defined minimum value, the tuning by the control unitcan be stopped. Then the tuning unit is providing an output voltage,which is a voltage having the stability of the ultra-stable DC voltagesource and the accuracy of the accurate DC voltage source and therebythis voltage can be considered to be tuned or calibrated. The outputvoltage provided by the tuning unit can be directly used as thereference voltage of the power supply or the reference voltage of thepower supply can be based on the output voltage of the tuning unit. Thenpreferably the output voltage of the tuning unit is supplied to anamplifier and the amplified voltage is the reference voltage provided bythe voltage supply. The provided reference voltage has been adjustedbased on the accurate voltage, which is provided to the comparator andis based on the supplied voltage of the accurate DC voltage source andhas due to this adjustment the same accuracy than the accurate voltage,which is provided to the comparator, which has the accuracy of theaccurate DC voltage source. But due to the circuit design of theinventive power supply it has now the stability of the ultra-stable DCvoltage source due the ultra-stable voltage applied to the tuning unit,which is provided based on the supplied voltage of the ultra-stable DCsource. This is in particularly correct, if any electrical componentlike amplifier have no influence on the accuracy of the accurate voltageprovided to the comparator and stability the ultra-stable voltageapplied to the tuning unit. Otherwise the performance of the inventivevoltage supply may be reduced. But nevertheless the inventive voltagesupply will provide a reference voltage of high accuracy and highstability.

When in the preferred embodiment of the inventive voltage supply theoutput voltage of the tuning unit and the voltage of the accuratevoltage source are provided to the comparator of the inventive powersupply directly and the signal provided from the comparator indicates,that the absolute difference of the output voltage of the tuning unit tothe voltage of the accurate DC voltage source is below a defined minimumvalue, the tuning by the control unit can be stopped. Then the tuningunit is providing an output voltage which is having the stability of theultra-stable DC voltage source and the accuracy of the accurate DCvoltage source and based on which the reference voltage of the powersupply is provided. Preferably the output voltage provided by the tuningunit can be used directly as the reference voltage provided by theinventive power supply. The average of the output voltage of the tuningunit is equal to the average of the voltage supplied by the accurate DCvoltage source. So when of the output voltage of the tuning unit is useddirectly as the reference voltage provided by the inventive power supplythe provided reference voltage has—apart from a difference smaller thedefined minimum value—the value of the voltage supplied by the accurateDC voltage source and the same accuracy. But due to the circuit designof the inventive power supply it has now the stability of theultra-stable DC voltage source.

Preferably the ratio of the defined minimum value, when the tuning ofthe tuning unit is stopped, to the nominal average value of the voltagesupplied by the accurate DC voltage source is below 100 ppm, preferablybelow 40 ppm, more preferably below 25 ppm and most preferably below 10ppm.

In another embodiment of the inventive power supply the tuning of thetuning unit by the control unit is a process which is minimising theabsolute difference of the voltage provided to the comparator, which isbased on the output voltage of the tuning unit, to the accurate voltage,which is provided to the comparator based on the supplied voltage of theaccurate DC voltage source below a defined minimum. The tuning of thetuning unit by the control unit is finished at the end of the processand then the tuning unit is providing an output voltage based on whichthe reference voltage of the inventive power supply can be provided. Asalready explained before, the reduction of the absolute difference ofthe voltage provided to the comparator, which is based on the outputvoltage of the tuning unit, to the accurate voltage, which is providedto the comparator based on the supplied voltage of the accurate DCvoltage source, results in a reference voltage of the inventive powersupply of at least high accuracy and stability and might have, dependingon the detailed circuit design of the inventive power supply, thestability of the ultra-stable DC voltage source and the accuracy of theaccurate DC voltage source.

As already mentioned before, the tuning unit of the inventive voltagesupply can be a digital to analogue converter. In such an embodiment ofthe inventive voltage supply the control unit of the voltage supplyprovides a digital signal to the digital to analogue converter to tunethe output voltage of the digital to analogue converter.

As also mentioned before, the tuning unit of the inventive voltagesupply can comprise one or more digital to analogue converters. In suchan embodiment of the inventive voltage supply the control unit of thevoltage supply provides a digital signal to each digital to analogueconverter to tune the output voltage of the digital to analogueconverters.

Preferably a digital to analogue converter of the tuning unit comprisesa resistor ladder network, in particular an R-2R resistor laddernetwork. Then the digital signal provided by the control unit, is adigital signal of a specific number of bits. Preferably the digitalsignal is a signal of at least 12 bits, particular preferably of atleast 16 bits and most preferably of at least 20 bits. When such adigital signal of several bits is provided as tuning input, the bits areswitching switches adding resistors in a parallel resistor network ofthe digital to analogue converter (DAC), which can an R-2R ladder DACcomprising a repeated cascaded structure of resistors having the valuesR and 2R. In this case each bit is binary weighted.

But the digital signal provided by the control unit can be also a signalof one single bit. Such a signal is in particular used, when a digitalto analogue converter of the tuning unit comprises a pulse widthmodulator and a low-pass filter. Then this one bit signal of the controlunit is provided to this digital to analogue converter via an input ofthe tuning unit.

When the control unit is providing a digital signal to the tuning unit,the control unit preferably comprises a processor or a programmablelogic circuit, like a Field Programmable Gate Array (FPGA) or ComplexProgrammable Logic Device (CPLD).

Preferably the control unit provides a digital signal to each digital toanalogue converter of the tuning unit according to the digital signalprovided by the comparator. The digital signals provided by the controlunit in an preferred embodiment are increasing the absolute value of theoutput voltage of the tuning unit if the absolute value of the accuratevoltage, which is provided to the comparator based on the suppliedvoltage of the accurate DC voltage source is higher than the absolutevalue of the voltage provided to the comparator, which is based on theoutput voltage of the tuning unit. In this case the digital signalprovided by the comparator indicates by a first value that the absolutevalue of the accurate voltage, which is provided to the comparator basedon the supplied voltage of the accurate DC voltage source is higher thanthe absolute value of the voltage provided to the comparator, which isbased on the output voltage of the tuning unit. The control unit reactson the first value of the digital signal provided by the comparator andprovides a digital signal to each digital to analogue converter of thetuning unit to increase the absolute value of the output voltage of thetuning unit. The digital signals provided by the control unit in thispreferred embodiment are decreasing the absolute value of the outputvoltage of the tuning unit if the absolute value of the accuratevoltage, which is provided to the comparator based on the suppliedvoltage of the accurate DC voltage source is lower than the absolutevalue of the voltage provided to the comparator, which is based on theoutput voltage of the tuning unit. In this case the digital signalprovided by the comparator indicates by a second value that the absolutevalue of the accurate voltage, which is provided to the comparator basedon the supplied voltage of the accurate DC voltage source is lower thanthe absolute value of the voltage provided to the comparator, which isbased on the output voltage of the tuning unit. The control unit reactson the second value of the digital signal provided by the comparator andprovides digital signals to each digital to analogue converter of thetuning unit to decrease the absolute value of the output voltage of thetuning unit.

When the control unit provides a digital signal to a tuning unit, whichis a digital to analogue converter, according to a digital signalprovided by the comparator, a digital signal is provided by the controlunit to increase in another preferred embodiment the absolute value ofthe output voltage of the digital to analogue converter if the absolutevalue of the accurate voltage, which is provided to the comparator basedon the supplied voltage of the accurate DC voltage source is higher thanthe absolute voltage provided to the comparator, which is based on theoutput voltage of the digital to analogue converter. In this case thedigital signal provided by the comparator indicates by a first valuethat the absolute value of the accurate voltage, which is provided tothe comparator based on the supplied voltage of the accurate DC voltagesource is higher than the absolute value of the voltage provided to thecomparator, which is based on the output voltage of the digital toanalogue converter. The control unit reacts on the first value of thedigital signal provided by the comparator and provides a digital signalto the digital to analogue converter to increase the absolute value ofits output voltage. A digital signal is provided by the control unit todecrease in this preferred embodiment the absolute value of the outputvoltage of the digital to analogue converter, if the value of theabsolute value of the accurate voltage, which is provided to thecomparator based on the supplied voltage of the accurate DC voltagesource is lower than the absolute value of the voltage provided to thecomparator, which is based on output voltage of the digital to analogueconverter. In this case the digital signal provided by the comparatorindicates by a second value that the absolute value of the accuratevoltage, which is provided to the comparator based on the suppliedvoltage of the accurate DC voltage source is lower than the absolutevalue of the voltage provided to the comparator, which is based onoutput voltage of the digital to analogue converter. The control unitreacts on the second value of the digital signal provided by thecomparator and provides a digital signal to the digital to analogueconverter to decrease the absolute value of its output voltage.

Preferably the increase and decrease of the output voltage of the tuningunit is reduced stepwise by the control unit providing accordinglydigital signals to the digital to analogue converters of the tuningunit. By this reduced change of the output voltage of the tuning unitthe absolute difference between the voltage provided to the comparator,which is based on output voltage of the tuning unit and the accuratevoltage, which is provided to the comparator based on the suppliedvoltage of the accurate DC voltage source is minimised.

When the tuning unit is a digital to analogue converter, also preferablythe increase and decrease of the output voltage of the digital toanalogue converter is reduced stepwise by the control unit providingaccordingly a digital signal. By this reduced change of the outputvoltage of the digital to analogue converter the absolute differencebetween the voltage provided to the comparator, which is based on outputvoltage of the digital to analogue converter and the accurate voltage,which is provided to the comparator based on the supplied voltage of theaccurate DC voltage source is minimised.

In many cases the reference voltage provided by the inventive powersupply is applied at at least one voltage amplifier, which is applyingan amplified voltage to the at least electrode. The reference voltageprovided by the inventive power supply can be applied at the at leastone voltage amplifier via a switch.

When the output voltage of the tuning unit is directly provided by theinventive voltage supply as reference voltage, the output voltage of thetuning unit can be applied at at least one voltage amplifier, which isapplying an amplified voltage to the at least one electrode. The outputvoltage of the tuning unit can then be applied at the at least onevoltage amplifiers via a switch.

In an embodiment of the inventive voltage supply an accurate voltage,which based on the supplied voltage of the accurate DC voltage source,can be also applied at the voltage amplifier via the switch, which is afurther component of the inventive voltage supply. The accurate voltageapplied at the switch can be in particular an amplified voltage of theaccurate voltage supplied by the accurate DC voltage source. Theaccurate voltage supplied by the accurate DC voltage source is suppliedto another amplifier provided in the inventive power supply, which isproviding the amplified voltage of the accurate voltage supplied by theaccurate DC voltage source. This amplified voltage is then applied atthe switch. In an embodiment of the inventive voltage supply theaccurate voltage, which is provided to the comparator based on thesupplied voltage of the accurate DC voltage source, can be applied alsoat the voltage amplifier via the switch, which is a further component ofthe inventive voltage supply.

In an embodiment of the inventive voltage supply also the suppliedvoltage of the accurate DC voltage source can be applied at the voltageamplifier via the switch, which is a further component of the inventivevoltage supply.

The reference voltage provided by the inventive power supply can be usedto supply a voltage to one or more electrodes of a mass spectrometer.

The one or more electrodes can be preferably one or more trappingelectrodes for trapping ions of an electrostatic trap and the referencevoltage applied to the one or more trapping electrodes is preferably atrapping voltage.

In particular the reference voltage provided by the inventive powersupply can be used to supply a voltage to the center electrode of anelectrostatic trap.

Typically to the electrode of the electrostatic trap is applied avoltage in the range of 0.7 kV to 12 kV. Typically to a center electrodeof the electrostatic trap is applied a voltage in the range of 1 kV to 8kV, preferably a voltage in the range of 2 kV to 7 kV.

In particular it can be an advantage to use the reference voltageprovided by the inventive power supply by amplifying the referencevoltage to provide an accurate and ultra-stable voltage to the centerelectrode of an electrostatic trap, in particular orbital trappingelectrostatic trap, which can be used as a mass analyser, such as theOrbitrap® mass analyser distributed by Thermo Fisher Scientific Inc. Thecenter electrode of such a mass analyser is for example disclosed in theinternational patent application WO96/30930, which content is herebyincorporated in this description.

The center electrode is preferably arranged radially central in anelectrostatic trap having a longitudinal axis and more preferablyarranged radially central and axially central in an electrostatic traphaving a longitudinal axis. But the center electrode can be alsoarranged only axially central in an electrostatic trap having alongitudinal axis.

The electrode of a mass spectrometer can be also an electrode of atime-of-flight mass spectrometer, in particular of a multi-reflectiontime-of-flight mass spectrometer. In multi-reflection time-of-flightmass spectrometer the reference voltage provided by the voltage supplycan be used to provide a voltage to mirror electrodes, preferably allmirror electrodes. Preferably the reference voltage is provided to avoltage amplifier which then provides a voltage in the range ofkilovolts (kV) to mirror electrodes, preferably all mirror electrodes,which are acting preferably as ion-optical mirrors. Typically the ionsin one type of a multi-reflection time-of-flight mass spectrometer arereflected between opposing ion-optical mirrors several times while theyare drifting along a drift direction. Due to the multi-reflection of theions at the ion-optical mirrors comprising several mirror electrodes,preferably each electrode has to be provided with an accurate andultra-stable voltage. Any instability could change the trajectory of theions which are oscillating between the reflecting ion-optical mirrors.Accordingly a changed flight-time of the analysed ions would result intime-of-flight mass spectra of lower resolving power or changing themass-to-charge-calibration of the detected time-of-flight mass spectra.

In an additional aspect, this disclosure also provides a massspectrometer comprising a voltage supply in accordance with theinvention, which is claimed in claim 28.

The ultra-stable DC voltage source of the inventive power supply haspreferably a voltage stability below 1 ppm over a time period of 24hours.

The ultra-stable DC voltage source of the inventive power supply haspreferably a voltage stability below 1 ppm over a temperature range of10° C.

The ultra-stable DC voltage source of the inventive power supply hasparticular preferably a voltage stability below 1 ppm over a time periodof 24 hours and a temperature range of 10° C.

The accurate DC voltage source of the inventive power supply haspreferably a production accuracy of its supplied voltage below 1000 ppm,preferably below 400 ppm, more preferably below 250 ppm and mostpreferably below 100 ppm.

The second object is solved by the method for calibrating a voltagesupply according to claim 23.

The method is calibrating a voltage supply of the present invention,which is providing a reference voltage to supply a voltage to at leastone electrode. The voltage supply can provide via a switch a voltage asthe reference voltage, which is based on the output voltage of itstuning unit, after the calibration. As already mentioned before, forexample the output voltage of the tuning unit can be used directly tosupply the reference voltage or the output voltage of the tuning unitcan be supplied to an amplifier and then the amplified voltage is thereference voltage provided by the voltage supply.

The method comprises the steps:

-   -   When the voltage supply is activated, a voltage provided based        on the output voltage of the tuning unit is applied to the        switch. But this voltage is not provided via the switch as the        reference voltage, which is used to supply a voltage to at least        one electrode.    -   The control unit of the voltage supply is tuning the tuning unit        according to the signal provided by the comparator to minimise        the absolute difference between the voltage based on the output        voltage of the tuning unit and the accurate voltage, which is        provided based on the supplied voltage of the accurate DC        voltage source compared by the comparator.    -   When the absolute difference between the voltages compared by        the comparator is below a defined minimum value and the tuning        by the control unit is stopped, the control unit submits a        switching signal to the switch, the switch receiving the        switching signal is actuated and then the voltage provided based        on the output voltage of the tuning unit applied to the switch        is provided by the voltage supply via the switch as the        reference voltage, which then is used to supply a voltage to at        least one electrode.

In general any voltage, which is based on the output voltage of thetuning unit, can be applied to the switch. This voltage can be derivedfrom the output voltage of the tuning unit by any appropriatearrangement known by a skilled person

In particular different voltages, which are based on the output voltageof the tuning unit, can be applied at the switch and compared by thecomparator. But in a preferred embodiment the same voltage, which isbased on the output voltage of the tuning unit, can be applied at theswitch and compared by the comparator. So the voltage provided based onthe output voltage of the tuning unit applied to the switch is thevoltage based on the output voltage of the tuning unit, which iscompared by the comparator.

The voltage provided based on the output voltage of the tuning unitapplied to the switch is only used as reference voltage, when the tuningprocess of the tuning unit has been finished. Then due to the smallabsolute difference between the voltages compared by the comparator thevoltage provided based on the output voltage of the tuning unit appliedto the switch has a high accuracy and high stability of an definedvoltage value, which is only preset by the electrical components of thepower supply, in particular the accurate DC voltage source and possiblypresent voltage amplifiers, preferably voltage amplifiers amplifying thevoltage supplied by the accurate DC voltage source and/or the outputvoltage of the tuning unit.

The second object can be also solved by the method for calibrating avoltage supply according to claim 25. Also the third second object canbe solved by the method for calibrating a voltage supply according toclaim 25.

The method is calibrating a voltage supply of the present invention,which is providing a reference voltage to supply a voltage to at leastone electrode. In the method, the inventive voltage supply is comprisinga switch, to which a voltage provided based on the output voltage of thetuning unit and an accurate voltage, which is provided based on thesupplied voltage of the accurate DC voltage source are applied.

The accurate voltage applied at the switch has preferably nearly thesame value as the reference voltage of the inventive voltage supply. Thevalue of the accurate voltage applied to the switch typically deviatesnot more than 1% from the value of the reference voltage of theinventive voltage supply, preferably not more than 1,000 ppm and inparticular preferably not more than 100 ppm.

As already mentioned before, the output voltage of the tuning unit canbe used directly to supply the reference voltage and applied to theswitch of the inventive voltage supply or the output voltage of thetuning unit can be supplied to an amplifier and then the amplifiedvoltage is applied to the switch of the inventive voltage supply and isthe reference voltage provided by the voltage supply.

These embodiments represent simple arrangements showing how a voltageprovided based on the output voltage of the tuning unit can be appliedto the switch. For example other arrangements known by a skilled personmight be used to provide a voltage, which is derived from the outputvoltage of the tuning unit, to the switch.

Also, the voltage supplied by the accurate DC voltage source can bedirectly applied to the switch of the inventive voltage supply or thevoltage supplied by the accurate DC voltage source can be supplied to anamplifier and then the amplified voltage is applied to the switch of theinventive voltage supply.

These embodiments represent simple arrangements showing how an accuratevoltage, which is provided based on the supplied voltage of accurate DCvoltage source, can be provided to the switch. For example otherarrangements known by a skilled person might be used to provide anaccurate voltage, which is derived from the supplied voltage of accurateDC voltage source, to the switch.

Via the switch, during an initial period, the accurate voltage, which isprovided based on the supplied voltage of the accurate DC voltage sourceis supplied to the voltage amplifier, while the voltage provided basedon the output voltage of the tuning unit applied at the switch is notsupplied via the switch to the amplifier. After tuning, the referencevoltage, which is then the voltage provided based on the output voltageof the tuning unit applied at the switch is applied to the voltageamplifier, which is supplying the voltage to at least one electrode. Themethod comprises the steps:

-   -   When the voltage supply is activated, a voltage provided based        on the output voltage of the tuning unit and an accurate        voltage, which is provided based on the supplied voltage of the        accurate DC voltage source are applied to the switch and the        voltage provided based on the output voltage of the tuning unit        applied to the switch is not connected via the switch with the        voltage amplifier and the accurate voltage applied to the switch        is connected via the switch with the voltage amplifier.    -   The control unit of the voltage supply is tuning the tuning unit        according to the signal provided by the comparator to minimise        the absolute difference between the voltage based on the output        voltage of the tuning unit and the accurate voltage, which is        provided based on the supplied voltage of the accurate DC        voltage source compared by the comparator;    -   When the absolute difference of the voltages compared by the        comparator is below a defined minimum value and the tuning by        the control unit is stopped, the control unit submits a        switching signal to the switch. The switch receiving the        switching signal is actuated and then the voltage provided based        on the output voltage of the tuning unit applied to the switch        is connected via the switch with the voltage amplifier as the        reference voltage and the accurate voltage applied to the switch        is disconnected by the switch from the voltage amplifier.

In general any voltage, which is based on the output voltage of thetuning unit, can be applied to the switch. This voltage can be derivedfrom the output voltage of the tuning unit by any appropriatearrangement known by a skilled person

In particular different voltages, which are based on the output voltageof the tuning unit, can be applied at the switch and compared by thecomparator.

In general any accurate voltage, which is provided based on the suppliedvoltage of the accurate DC voltage source, can be applied to the switch.This voltage can be derived from the supplied voltage of the accurate DCvoltage source by any appropriate arrangement known by a skilled person

In particular different accurate voltages, which are provided based onthe supplied voltage of the accurate DC voltage source, can be appliedat the switch and compared by the comparator.

The same voltage provided based on the output voltage of the tuning unitcan be applied to the switch and the comparator. So the voltage providedbased on the output voltage of the tuning unit applied to the switch isthe voltage based on the output voltage of the tuning unit, which iscompared by the comparator.

The same accurate voltage provided based on the supplied voltage of theaccurate DC voltage source can be applied to the switch and thecomparator. So the accurate voltage, which is applied to the switch, isthe accurate voltage, which is compared by the comparator.

In a preferred embodiment the same voltage provided based on the outputvoltage of the tuning unit and the same accurate voltage provided basedon the supplied voltage of the accurate DC voltage source can be appliedto the switch and the comparator.

In a preferred embodiment of the method the inventive voltage supplycomprises a switch, to which the output voltage of the tuning unit andthe voltage of the accurate DC voltage source are applied. Thecomparator of the inventive voltage supply of this embodiment iscomparing the output voltage of the tuning unit and the voltage of theaccurate DC voltage source. Via the switch, during an initial period,the accurate DC voltage source is connected to the voltage amplifier,while the output voltage of the tuning unit is not connected to theamplifier. After tuning, the reference voltage provided by the outputvoltage of the tuning unit is applied to a voltage amplifier, which issupplying the voltage to at least one electrode. The method comprisesthe steps:

-   -   When the voltage supply is activated, the output voltage of the        tuning unit and the supplied voltage of the accurate DC voltage        source are applied to the switch and the output voltage of the        tuning unit applied to the switch is not connected via the        switch with the voltage amplifier and the supplied voltage of        the accurate DC voltage source is connected via the switch with        the voltage amplifier.    -   The control unit of the voltage supply is tuning the tuning unit        according to the signal provided by the comparator to minimise        the absolute difference between the output voltage of the tuning        unit and the supplied voltage of the accurate DC voltage source.    -   When the absolute difference of the voltages compared by the        comparator is below a defined minimum value and the tuning by        the control unit is stopped, the control unit submits a        switching signal to the switch. The switch receiving the        switching signal is actuated and then the output voltage of the        tuning unit is connected via the switch with the voltage        amplifier as the reference voltage and the supplied voltage of        the accurate DC voltage source is disconnected by the switch        from the voltage amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of an inventive voltage supplyschematically.

FIG. 2 shows a second embodiment of an inventive voltage supplyschematically.

FIG. 3 shows a third embodiment of an inventive voltage supplyschematically comprising digital components.

FIG. 4 shows a fourth embodiment of an inventive voltage supplyschematically.

FIG. 5 shows a fifth embodiment of an inventive voltage supplyschematically.

FIG. 6 shows a sixth embodiment of an inventive voltage supplyschematically.

FIG. 7 shows a seventh embodiment of an inventive voltage supplyschematically.

FIG. 8 shows an eighth embodiment of an inventive voltage supplyschematically.

FIG. 9 shows a ninth embodiment of an inventive voltage supplyschematically.

FIG. 10 shows a tenth embodiment of an inventive voltage supplyschematically.

FIG. 11 shows an eleventh embodiment of an inventive voltage supplyschematically.

FIG. 12 shows a twelfth embodiment of an inventive voltage supplyschematically.

FIGS. 13 a-13 d show details of the third embodiment of an inventivevoltage supply.

FIG. 14 shows time stability behaviour of the reference voltage providedby an accurate voltage supply and of the reference voltage provided byan inventive voltage supply.

FIG. 15 shows the temperature behaviour of the reference voltageprovided by an accurate voltage supply and of the reference voltageprovided by an inventive voltage supply.

FIGS. 16 a and 16 b show a first embodiment of a multi-reflectiontime-of-flight mass analyser, to which a voltage can be applied usingthe inventive voltage supply.

FIG. 17 shows a second embodiment of a multi-reflection time-of-flightmass analyser, to which a voltage can be applied using the inventivevoltage supply.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of an inventive voltage supply 2.Schematically are shown the main components of the voltage supply 2,which are essential for the invention.

The inventive voltage supply 2 comprises two different DC voltagesources, an ultra-stable DC voltage source 4 (V1) and an accurate DCvoltage source 6 (V2).

The ultra-stable DC voltage source 4 provides a very stable outputvoltage. The stability of the output voltage is provided over the timeand a specific temperature range. Typically the output voltage provideby a DC ultra-stable voltage source 4 has a voltage stability below 5ppm, preferably below 1 ppm, more preferably below 0.5 ppm andparticular preferably below 0.3 ppm. The stability of the output voltageis provided typically over more than 12 hours, preferably over more than24 hours, more preferably over more than 48 hours and particular overmore than 96 hours. The stability of the output voltage is providedtypically over a temperature range of more than 10° C., preferably overmore than 15° C., more preferably over more than 20° C. and particularover more than 25° C.

For example in the first embodiment of the inventive voltage supply 2 anultra-stable DC voltage source 4 can be used with a voltage stabilitybelow 2 ppm over more than 24 hours and a temperature range of more than10° C.

The accurate DC voltage source 6 provides a very accurate DC outputvoltage with a production accuracy typically below 1,000 ppm, preferablybelow 400 ppm, more preferably below 250 ppm and most preferably below100 ppm.

For example in the first embodiment of the inventive voltage supply 2 anaccurate DC voltage source 6 can be used with a production accuracybelow 500 ppm.

In the first embodiment of the inventive voltage supply 2 the voltagesupplied by the ultra-stable DC source 4 has a higher absolute valuethan the voltage supplied by the accurate DC voltage source 6.

Typically in the first embodiment of the inventive voltage supply 2 theabsolute value of the voltage of the ultra-stable DC source 4 is atleast 2% higher than the absolute value of the voltage of the accurateDC voltage source 6. Preferably the absolute value of the voltage of theultra-stable DC source 4 is at least 10% higher than the absolute valueof the voltage of the accurate DC voltage source 6. More preferably theabsolute value of the voltage of the ultra-stable DC source 4 is atleast 25% higher than the absolute value of the voltage of the accurateDC voltage source 6.

Typically the absolute value of the voltage of the ultra-stable DCsource 4 is not more than 500% higher than the absolute value of thevoltage of the accurate DC voltage source 6. Preferably the absolutevalue of the voltage of the ultra-stable DC source 4 is not more than200% higher than the absolute value of the voltage of the accurate DCvoltage source 6. More preferably the absolute value of the voltage ofthe ultra-stable DC source 4 is not more than 100% higher than theabsolute value of the voltage of the accurate DC voltage source 6.

Typically both voltages sources of the inventive voltage supply, theultra-stable DC source 4 and the accurate DC voltage source 6 providevoltages with an absolute value in the range of 0.5 V to 100 V,preferably in the range of 2 V and 20 V and more preferably in the rangeof 2.5 V to 10 V. For example in the first embodiment of the inventivevoltage supply 2 an ultra-stable DC source 4 can be used, which providean average voltage of 9 V and an accurate DC voltage source 6 whichprovide a nominal voltage of 7 V. Then the inventive voltage supply 2will provide a reference voltage of 7 V.

The inventive voltage supply 2 also comprises a tuning unit 8, which canbe a tunable voltage divider. A tunable voltage divider comprises atleast one resistor, preferably a network of connected resistors. In thefirst embodiment of the inventive voltage supply 2 the voltage of the DCultra-voltage source 4 is applied to two (input) connectors (not shown)of the tuning unit 8 shown by the arrow 10. The tuning unit 8 comprisesan output connector (not shown) to which the tuning unit 8 provides anoutput voltage. If the tuning unit 8 is a tunable voltage divider, theoutput voltage is provided by taping the voltage from the resistor orthe resistor network of the tunable voltage divider.

The tuning unit 8 provides an output voltage which can be adjusted.

The tuning unit 8 of the inventive voltage supply 2 can be tunedanalogue or digitally. In an analogue tuning unit the tuning can beexecuted also by an electrical component of the electrical circuit or amechanical component of the tuning unit 8, due to which a tuned outputvoltage is provided to the (output) connector of the tuning unit 8. Ifthe tuning unit is digitally tuned, a digital signal of one or severalbits is provided to at least one tuning input (not shown) of the tuningunit 8.

In the first embodiment of the inventive voltage supply 2 the outputvoltage of the tuning unit 8 is the reference voltage provided by thevoltage supply 2.

The inventive voltage supply 2 also comprises a comparator 12.Preferably the comparator 12 comprises an operational amplifier which ismore preferably a differential amplifier and in particular preferably ahigh-gain differential amplifier which is providing a digital signal.

In the first embodiment of the inventive voltage supply 2 the outputvoltage of the tunable voltage divider 8 shown by the arrow 14 and thevoltage of the accurate voltage source 6 shown by the arrow 16 areprovided to the comparator 12 of the inventive power supply 2, whichcompares the voltage signals. The comparator 12 provides an outputsignal which is resulting from the comparison of these two inputvoltages. The output signal can be a signal which is equal orproportional to the difference of the compared voltages. The outputsignal which is resulting from the comparison of these two inputvoltages can be also a digital signal which has only the output signals0 and 1. By these signals it is only indicated which of the two inputvoltages has the higher value.

For such a kind of comparator can be used a high-gain differentialamplifier, which is providing a very low or high voltage, which are thenidentified as signal 0 and 1.

The inventive power supply 2 comprises also a control unit 18.

The signal provided by the comparator 12, which results in the firstembodiment of the inventive voltage supply 2 from the comparison of theoutput voltage of the tuning unit 8 and the voltage of the accuratevoltage source 6 is provided to the control unit 18 as input signalshown by arrow 20.

The control unit 18 in the first embodiment of the inventive voltagesupply 2 is tuning the tuning unit 8 according to the signal provided bythe comparator 12 to minimise the difference between the output voltageof the tuning unit 8 and the voltage of the accurate DC voltage source6. To achieve this, the control unit 18 provides an output signal, whichis provided to the tuning unit 8 shown by arrow 22.

When the signal provided from the comparator 12 indicates, that thetotal difference of the output voltage of the tuning 8 to the voltage ofthe accurate DC voltage source 6 is below a defined minimum value, thetuning by the control unit 18 will be stopped. Preferably the ratio ofthe defined minimum value, when the tuning of the tunable voltagedivider 8 is stopped, to the nominal average value of the voltagesupplied by the accurate DC voltage source 6 is below 500 ppm,preferably below 200 ppm, more preferably below 50 ppm and mostpreferably below 10 ppm.

For example in the first embodiment of the inventive voltage supply 2the defined minimum value can be 140 μV. For a nominal voltage theaccurate DC voltage source 6 of 7 V then the ratio of the definedminimum value to the nominal average value of the voltage supplied bythe accurate DC voltage source 6 is 20 ppm.

Then the tuning unit 8 is providing an output voltage, which is in thefirst embodiment of the inventive voltage supply 2 a reference voltagehaving the stability of the ultra-stable DC voltage source 4 and theaccuracy of the accurate DC voltage source 6. The average of the outputvoltage of the tuning unit divider 8 is equal to the average of thevoltage supplied by the accurate DC voltage source 6. So the providedreference voltage has—apart from a difference smaller the definedminimum value—the value of the voltage supplied by the accurate DCvoltage source 6 and the same accuracy. But due to the inventive powersupply 2 it has now the stability of the ultra-stable DC voltage source4.

If the signal provided by the comparator 12 which is provided to thecontrol unit 18 is equal or proportional to the difference of thecompared voltages, according to the detected voltage difference thecontrol unit 18 reacts and provides a signal to the tuning unit 8 toincrease or decrease its output voltage according to the detectedvoltage difference. In the first embodiment of the inventive voltagesupply 2 the difference of the output voltage of the tuning unit 8 andthe voltage of the accurate DC voltage source 6 is reduced.

The output signal provided by the control unit 18 (arrow 22) in thefirst embodiment of the inventive voltage supply 2 is increasing theoutput voltage of the tunable voltage divider 8 if the value of thevoltage of the DC accurate voltage source 6 is higher than the outputvoltage of the tunable voltage divider 8.

In particular if the comparator 12 in the first embodiment of theinventive voltage supply 2 is providing a digital signal, then thedigital signal provided by the comparator 12 indicates by a first valuethat the voltage of the accurate DC voltage source 6 is higher than theoutput voltage of the tuning unit 8. The control unit reacts on thefirst value of the digital signal provided by the comparator andprovides a signal to the tuning unit 8 to increase its output voltage.

The output signal provided by the control unit 18 in the firstembodiment of the inventive voltage supply 2 is decreasing the outputvoltage of the tuning unit 8 if the value of the voltage of the accurateDC voltage source 6 is lower than the output voltage of the tunablevoltage divider 8.

In particular if the comparator 12 in the first embodiment of theinventive voltage supply 2 is providing a digital signal, then thedigital signal provided by the comparator 12 indicates by a second valuethat the voltage of the accurate DC voltage source 6 is lower than theoutput voltage of the tuning unit 8. The control unit reacts on thesecond value of the digital signal provided by the comparator 12 andprovides a signal to the tuning unit 8 to decrease its output voltage.

Preferably the increase and decrease of the output voltage of the tuningunit 8 is reduced stepwise by the control unit 18 providing accordinglya signal to the tuning unit 8.

By this reduced change of the output voltage of the tuning unit 8 in thefirst embodiment of the inventive voltage supply 2 the absolutedifference between the output voltage of the tuning unit 8 and thevoltage of the accurate DC voltage source 6 is minimised.

In the first embodiment of the FIG. 1 the output voltage of the tuningunit 8 of the inventive voltage supply 2 is applied as reference voltageat a voltage amplifier 24 shown by arrow 26, which is applying anamplified voltage to an electrode 28 shown by arrow 30. The outputvoltage of the tuning unit 8 is applied to the voltage amplifier 24 viaa switch 32. Accordingly the output voltage of the tuning unit 8 isdirectly applied to the switch 32 shown by arrow 34.

The switch 32 will connect in the first embodiment of the inventivevoltage supply 2 the output voltage of the tuning unit 8 with thevoltage amplifier 24 at a defined time after the voltage supply 2 hasbeen activated. Preferably the time delay to connect the output voltageof the tuning unit 8 with the voltage amplifier 24 is chosen in that waythat the tuning of the tuning unit 8 by the control unit 18 has beenfinished before the output voltage of the tuning unit 8 is connectedwith the voltage amplifier 24. Then the output voltage of the tuningunit 8 is a reference voltage, having the accuracy of the accurate DCvoltage source 6 and stability of the ultra-stable DC voltage source 4,which is applied to the voltage amplifier 24. Then the voltage amplifier24 is supplying a voltage of high accuracy and stability based on thereference voltage provided by the power supply 2 to the electrode 28.

In a not shown further embodiment of the inventive voltage supply 2 theswitch 32 is a component of the inventive voltage supply 2 and thecontrol unit 18 is submitting a switching signal to the switch 32, whenthe tuning of the tuning unit 8 by the control unit 18 is stopped. Thenthe switch 32 is actuated due to the received switching signal and avoltage provided based on the output voltage of the tuning unit 8applied to the switch is provided by the inventive voltage supply 2 viathe switch 32 as the reference voltage.

When the inventive voltage supply 2 has the same components as the firstembodiment of the inventive voltage supply 2 and further the controlunit 18 is connected with the switch 32 to submit a switching signal tothe switch 32 and the switch 32 is actuated due to the receivedswitching signal submitted after the tuning of the tuning unit 8, theoutput voltage of the tuning unit 8 is connected after the actuation ofthe switch with the voltage amplifier 24. Then output voltage of thetuning unit 8 is applied at the voltage amplifier 24 as an ultra-stableand accurate reference voltage.

FIG. 2 shows a second embodiment of an inventive voltage supply 2.Schematically are shown the main components of the voltage supply 2,which are essential for the invention. For components of the inventivevoltage supply 2 of the second and all further embodiments are used thesame reference signs as in the preceding embodiments, if the samecomponents are used in the embodiments.

Because the most components are in following embodiments the same as inthe preceding embodiments, these components are not described for thefollowing embodiments and it is referred to the description of thesecomponents regarding FIG. 1 or other preceding embodiments and theirrelated Figures.

The main difference of the first and second embodiment is that theswitch 32 is a component of the voltage supply 2 in the secondembodiment. The switch 32 is also able to connect the output voltage ofthe tuning unit 8 applied to the switch 32 with the voltage amplifier24, which is applying an amplified voltage to an electrode 28. But thevoltage of the accurate DC voltage source 6 is also provided to theswitch 32 shown by arrow 36.

An additional difference of the first and second embodiment is that thecontrol unit 18 also provides a switching signal to the switch 32 shownby the arrow 38.

The second embodiment of the voltage supply 2 is able to execute thefollowing calibration method of the voltage supply 2 to provide areference voltage of high accuracy and stability.

When the voltage supply 2 is activated, in a first step the switch 32 isconnecting the accurate DC voltage source 6 with the voltage amplifier24, which is applying an amplified voltage to an electrode 28. In thisstage of the calibration method the nominal voltage of the accurate DCvoltage source 6 is already applied at the voltage amplifier 24. So theapplied voltage is already accurate, but has limited stability accordingto the stability of the accurate DC voltage source 6.

In this first step is the output voltage of the tuning unit 8 is notconnected via the switch 32 with the voltage amplifier 24.

As already described for the first embodiment in detail, the controlunit 18 of the voltage supply 2 is tuning in the second step the tuningunit 8 according to the signal provided by the comparator 12 to minimisethe absolute difference between the output voltage of the tuning unit 8and the voltage of the accurate DC voltage source 6.

When the signal provided from the comparator 12 indicates, that theabsolute difference of the output voltage of the tuning unit 8 to thevoltage of the accurate DC voltage source 6 is below a defined minimumvalue, the tuning by the control unit 18 will be stopped. Then thecontrol unit submits in a third step a switching signal to the switch 32shown by the arrow 38. When the switch 32 has received the switchingsignal, the switch 32 is actuated and then the output voltage of thetuning unit 8 is connected with the voltage amplifier 24 and the voltagesupplied by the accurate DC voltage source 6 is disconnected by theswitch from the voltage amplifier 24. Then the output voltage of thetuning unit 8 is a reference voltage, having the accuracy of theaccurate DC voltage source 6 and stability of the ultra-stable DCvoltage source 4, which is applied at the voltage amplifier 24. Then thevoltage amplifier 24 is supplying a voltage of high accuracy andstability based on the to the reference voltage provided by the powersupply 2 to the electrode 28.

So in the second embodiment of the inventive voltage supply 2 at firstthe output voltage of the tuning unit 8 is calibrated to provide theintended reference voltage with high accuracy and stability and when thecontrol circuit 18 of the voltage supply 2 has this achieved, the switch32 is actuated to provide the optimized reference voltage to the voltageamplifier 24. During this calibration the nominal voltage of theaccurate DC voltage source 6 is applied to the voltage amplifier 24having a limited stability.

This limited stability of the voltage applied to the voltage amplifier24 during the tuning period of the reference voltage with high accuracyand stability is uncritical, because the tuning period is typically inthe range of some seconds, while the warm up of an activated voltagesupply of the electrode typically providing a high voltage in the kVrange takes a much longer time, typically in the range of 30 to 60minutes. And just if an instrument is switched off only for a shortperiod, the electronics supplying the voltage to the electrode requiressome time, typically in the range of several seconds to a few minutes,to be stabilised. In this phase it is helpful, when accurate DC voltagesource 6 is providing a voltage having already the value of the requiredreference voltage, because the electrical components, in particularresistors are already supplied with the right power. That in this phasethe voltage provided by the accurate DC voltage source 6 has a limitedstability is not relevant.

FIG. 3 shows a third embodiment of an inventive voltage supply 2.Schematically are shown the main components of the voltage supply 2,which are essential for the invention. For components of the inventivevoltage supply 2 of the third embodiment are used the same referencesigns as in the first embodiment of FIG. 1 or second embodiment of FIG.2 , if the same components are used.

The voltage supply 2 of the third embodiment comprises in principle thesame components as the second embodiment, but for several componentsdigital technology is used, which can improve the functionality of theinventive voltage supply 2 further.

The inventive voltage supply 2 of the third embodiment comprises twodifferent DC voltage sources, an ultra-stable DC voltage source 4 (V1)and an accurate DC voltage source 6 (V2).

The parameters of such ultra-stable DC voltage source 4 (V1) and anaccurate DC voltage source 6 (V2) are the same as described for thefirst embodiment. Also the relation of the absolute values of thevoltages of these DC voltages is the same as described for the firstembodiment.

For example in the third embodiment of the inventive voltage supply 2 anultra-stable DC source 4 can be used, which provides an average voltageof 7 V and an accurate DC voltage source 6 which provide a nominalvoltage of 5 V. Then the inventive voltage supply 2 will provide areference voltage of 5 V.

The third embodiment of the inventive voltage supply 2 comprises atuning unit, which is a digital to analogue converter (DAC) 108. Adigital signal of several bits is provided to the digital input of thedigital to analogue converter (DAC) 108 for digitally tuning the digitalto analogue converter (DAC) 108.

The voltage of the ultra-stable DC voltage source 4 is applied at two(input) connectors (not shown) of the digital to analogue converter(DAC) 108 shown by the arrow 10. The digital to analogue converter (DAC)108 comprises an output connector (not shown) to which the digital toanalogue converter (DAC) 108 provides an output voltage by tapping thevoltage from the resistor network of digital to analogue converter (DAC)108.

The output voltage of the digital to analogue converter (DAC) 108 is thereference voltage provided by the voltage supply 2.

The inventive voltage supply 2 also comprises a comparator 12.Preferably the comparator 12 comprises an operational amplifier which isa differential amplifier.

The output voltage of the digital to analogue converter (DAC) 108 shownby the arrow 14 and the voltage of the accurate voltage source 6 shownby the arrow 16 are provided to the comparator 12 of the inventive powersupply 2, which compares the voltage signals. The comparator 12 providesan output signal which is resulting from the comparison of these twoinput voltages. The output signal which is resulting from the comparisonof these two input voltages is a digital signal which has only theoutput signal 0 and 1. By these signals it is only indicated which ofthe two input voltages has the higher value.

For such a kind of comparator can be used a high-gain differentialamplifier, which is providing a very low or high voltage, which are thenidentified as signal 0 and 1.

The inventive voltage supply 2 comprises also a control unit, which isprocessor 118. A computer program can be executed by the processor tooperate the voltage supply 2 in accordance with the described steps.

The signal provided by the comparator 12, which results from thecomparison of the output voltage of digital to analogue converter (DAC)108 and the voltage of the accurate voltage source 6, is provided to theprocessor 118 as input signal shown by arrow 20.

The processor 118 is tuning the digital to analogue converter (DAC) 108according to the digital signal provided by the comparator 12 tominimise the absolute difference between the output voltage of thedigital to analogue converter (DAC) 108 and the voltage of the accurateDC voltage source 6. To achieve this, the processor 118 provides adigital output signal, which is provided to the digital to analogueconverter (DAC) 108 shown by arrow 22.

The output signal provided by the processor 118 (arrow 22) is increasingthe output voltage of the digital to analogue converter (DAC) 108 if thevalue of the voltage of the accurate DC voltage source 6 is higher thanthe output voltage of the digital to analogue converter (DAC) 108.

When the digital signal provided by the comparator 12 indicates by afirst value that the voltage of the accurate DC voltage source 6 ishigher than the output voltage of the digital to analogue converter(DAC) 108, the processor 118 reacts on the first value of the digitalsignal provided by the comparator 12 and provides a digital signal tothe digital to analogue converter (DAC) 108 to increase its outputvoltage.

The output signal provided by processor 118 is decreasing the outputvoltage of the digital to analogue converter (DAC) 108 if the value ofthe voltage of the DC accurate voltage source 6 is lower than the outputvoltage of the digital to analogue converter (DAC) 108.

When the digital signal provided by the comparator 12 indicates by asecond value that the voltage of the accurate DC voltage source 6 islower than the output voltage of the digital to analogue converter (DAC)108, the processor 118 reacts on the second value of the digital signalprovided by the comparator 12 and provides a digital signal to thedigital to analogue converter (DAC) 108 to decrease its output voltage.

Preferably the increase and decrease of the output voltage of thedigital to analogue converter (DAC) 108 is reduced stepwise by theprocessor 118 providing accordingly a signal to the digital to analogueconverter (DAC) 108. By this reduced change of the output voltage of thedigital to analogue converter (DAC) 108 the absolute difference betweenthe output voltage of the digital to analogue converter (DAC) 108 andthe voltage of the accurate DC voltage source 6 is minimised.

Then the digital to analogue converter (DAC) 108 is providing an outputvoltage, which is a reference voltage having the stability of theultra-stable DC voltage source 4 and the accuracy of the accurate DCvoltage source 6. The average of the output voltage of the digital toanalogue converter (DAC) 108 is equal to the average of the voltagesupplied by the accurate DC voltage source 6. So the provided referencevoltage has—apart from a small difference—the value of the voltagesupplied by the accurate DC voltage source 6 and the same accuracy. Butdue to the inventive power supply 2 it has now the stability of theultra-stable DC voltage source 4.

In a preferred embodiment the digital to analogue converter 108comprises a resistor ladder network, in particular an R-2R resistorladder network. The digital signal provided by the processor 118 in thispreferred embodiment is a digital signal of a specific number of bits.Preferably the digital signal is a signal of at least 16 bits,particular preferably of at least 20 bits. Based on the digital signalprovided by the comparator 12, the processor is applying the method ofsuccessive approximation to minimise the absolute difference between theoutput voltage of the digital to analogue converter (DAC) 108 and thevoltage of the accurate DC voltage source 6. In this method according tothe bits of the digital signal provided by the processor 118 to thedigital to analogue converter (DAC) 108 the output voltage of thedigital to analogue converter (DAC) 108 is determined by the R-2Rresistor ladder network. The resistor network is dividing with each setbit (value 1) the provided voltage of the ultra-stable DC voltage source4 into equal shares. So the first bit is dividing the voltage to theshare ½, the second is dividing the remaining voltage into shares of ¼,the third is dividing the remaining voltage into shares of ⅛ and soforth.

Before the approximation all bits of the digital signal are set to 0(alternatively they can all be set to 1). When the comparator 12 at thebeginning of the approximation is providing for the first time a digitalsignal, which is indicating if the voltage of the accurate DC voltagesource 6 is higher or lower than the output voltage of the digital toanalogue converter (DAC) 108, the first bit is accordingly set by theprocessor to adjust accordingly the output voltage of the digital toanalogue converter (DAC) 108. Normally in this first iteration step thefirst bit is set to 1 and accordingly the output voltage of the digitalto analogue converter (DAC) 108 is the half of the voltage of theultra-stable DC voltage source 4. Then the comparator 12 in the nextiteration is providing a second digital signal, which is indicating ifthe voltage of the accurate DC voltage source 6 is higher or lower thanthe output voltage of the digital to analogue converter (DAC) 108. Ifnow the voltage of the output voltage of the digital to analogueconverter (DAC) 108 too high due to setting the bit before to 1, the bitis set back and the second bit is set to 1. Otherwise the second bit isset to 1 and the first bit remains unchanged. Then the comparator 12 inthe next iteration is providing a third digital signal, which isindicating if the voltage of the accurate DC voltage source 6 is higheror lower than the output voltage of the digital to analogue converter(DAC) 108. If now the voltage of the output voltage of the digital toanalogue converter (DAC) 108 too high due to setting the (second) bitbefore to 1, the bit is set back and the third bit is set to 1.Otherwise only the third bit is set to 1 and the second bit remainsunchanged.

Over the iteration steps of the method each of the bits of the digitalsignal provided by the processor 118 to the digital to analogueconverter (DAC) 108 is set and the output voltage of the digital toanalogue converter (DAC) 108 is correctly adjusted to the voltage of theaccurate DC voltage source 6. The accuracy of this adjustment isincreasing fast and so the ratio of the remaining absolute differencebetween output voltage of the digital to analogue converter (DAC) 108and the voltage of the accurate DC voltage source 6 at the end of theapproximation to the nominal average value of the voltage of theaccurate DC voltage source 6 is for a 20 bit signal provided by theprocessor 118 to the digital to analogue converter (DAC) 108 1 ppm, fora 16 bit signal 15 ppm and a 14 bit signal 61 ppm. Depending on theproperties of the comparator the remaining absolute difference could bealso bigger than these values, but this is not relevant as long as thereference voltage has now the required stability.

In the embodiment of the FIG. 3 the output voltage of the digital toanalogue converter (DAC) 108 of the inventive voltage supply 2 isapplied as reference voltage at a voltage amplifier 24 shown by arrow26, which is applying an amplified voltage to an electrode 28 shown byarrow 30. The output voltage of the digital to analogue converter (DAC)108 is applied at the voltage amplifier 24 via a switch 32 after thetuning process has been finished. Accordingly the output voltage of thedigital to analogue converter (DAC) 108 is directly applied to theswitch 32 shown by arrow 34.

The voltage of the accurate DC voltage source 6 is also provided to theswitch 32 shown by arrow 36.

The processor 118 provides a switching signal to the switch 32 shown bythe arrow 38.

The third embodiment of the voltage supply 2 is able to execute thefollowing calibration method of the voltage supply 2 to provide areference voltage of high accuracy and stability.

When the voltage supply 2 is activated, in a first step the switch 32 isconnecting the accurate DC voltage source 6 with the voltage amplifier24, which is applying an amplified voltage to an electrode 28. In thisstage of the calibration method the nominal voltage of the accurate DCvoltage source 6 is already applied at the voltage amplifier 24. So theapplied voltage is already accurate, but has limited stability accordingto the stability of the accurate DC voltage source 6.

In this first step is the output voltage of the digital to analogueconverter (DAC) 108 not connected via the switch 32 with the voltageamplifier 24.

The processor 118 of the voltage supply 2 is tuning in the second stepthe digital to analogue converter (DAC) 108 according to the signalprovided by the comparator 12 to minimise the absolute differencebetween the output voltage of the digital to analogue converter (DAC)108 and the voltage of the accurate DC voltage source 6 as describedbefore preferably using the method of successive approximation.

When the tuning by the processor 118 has been finished, which means thatall bits are set when the method of successive approximation is used,the processor 118 submits in a third step a switching signal to theswitch 32 shown by the arrow 38. When the switch 32 has received theswitching signal, the switch is actuated and then the output voltage ofthe digital to analogue converter (DAC) 108 is connected with thevoltage amplifier 24 and the supplied voltage of the accurate DC voltagesource 6 is disconnected by the switch from the voltage amplifier 24.Then the output voltage of the digital to analogue converter (DAC) 108is a reference voltage, having the accuracy of the accurate DC voltagesource 6 and stability of the ultra-stable DC voltage source 4, which isapplied at the voltage amplifier 24. Then the voltage amplifier 24 issupplying a voltage of high accuracy and stability based on thereference voltage provided by the voltage supply 2 to the electrode 28.

So in the third embodiment of the inventive voltage supply 2 at firstthe output voltage of the digital to analogue converter (DAC) 108 iscalibrated to provide the intended reference voltage with high accuracyand stability and when the control circuit of the voltage supply 2 hasthis achieved, the switch 32 is actuated to provide the optimisedreference voltage to the voltage amplifier 24. During this calibrationthe nominal voltage of the accurate DC voltage source 6 is applied atthe voltage amplifier 24 which has a limited stability.

FIG. 4 shows a fourth embodiment of the inventive power supply 2.

In this embodiment, in which in general all components are the same asin the first embodiment, the voltage supplied by the ultra-stable DCvoltage source 4 is not applied directly to (input) connectors (notshown) of the tuning unit 8. The voltage of the ultra-stable DC voltagesource 4 is applied to a voltage amplifier 180—named in the furtherspecification ultra-stable voltage amplifier 180— shown by the arrow182. The amplified voltage provided at the output of the ultra-stablevoltage amplifier 180 is then provided to two (input) connectors (notshown) of the tuning unit 8 shown by the arrow 10 as an example of anultra-stable voltage which is based on the supplied voltage of theultra-stable voltage source 4.

In the fourth embodiment of the inventive voltage supply 2 the amplifiedvoltage provided at the output of the ultra-stable voltage amplifier 180has a higher absolute value than the voltage supplied by the accurate DCvoltage source 6. The voltage supplied by the ultra-stable DC voltagesource 4 can have a lower absolute value than the voltage supplied bythe accurate DC voltage source 6.

Typically in the fourth embodiment of the inventive voltage supply 2 theabsolute value of the amplified voltage provided at the output of theultra-stable voltage amplifier 180 is at least 2% higher than theabsolute value of the voltage of the accurate DC voltage source 6.Preferably the absolute value of the amplified voltage provided at theoutput of the ultra-stable voltage amplifier 180 is at least 10% higherthan the absolute value of the voltage of the accurate DC voltage source6. More preferably the absolute value of the amplified voltage providedat the output of the ultra-stable voltage amplifier 180 is at least 25%higher than the absolute value of the voltage of the accurate DC voltagesource 6.

Typically the absolute value of the amplified voltage provided at theoutput of the ultra-stable voltage amplifier 180 is not more than 500%higher than the absolute value of the voltage of the accurate DC voltagesource 6. Preferably the absolute value of the amplified voltageprovided at the output of the ultra-stable voltage amplifier 180 is notmore than 200% higher than the absolute value of the voltage of theaccurate DC voltage source 6. More preferably the absolute value of theamplified voltage provided at the output of the ultra-stable voltageamplifier 180 is not more than 100% higher than the absolute value ofthe voltage of the accurate DC voltage source 6.

For example in the fourth embodiment of the inventive voltage supply 2an ultra-stable DC source 4 can be used, which provide an averagevoltage of 4 V and an accurate DC voltage source 6 which provide anominal voltage of 5 V. The ultra-stable voltage amplifier 180 is thenamplifying the average voltage of 4 V provided by the ultra-stable DCsource 4 to an amplified voltage of 7 V provided to two (input)connectors (not shown) of the tuning unit 8. Then the inventive voltagesupply 2 will provide a reference voltage of 5 V.

Because all other components of the first and fourth embodiment of theinventive voltage supply are the same, the tuning unit 8 of the fourthembodiment of the inventive voltage supply 2 is providing an outputvoltage, which is the reference voltage provided by the fourthembodiment of the inventive voltage supply 2 having the stability of theultra-stable DC voltage source 4 and the accuracy of the accurate DCvoltage source 6. This is in particularly correct, if the ultra-stablevoltage amplifier 180 has no influence on stability of the amplifiedvoltage applied to the tuning unit 8, so that the amplified voltageprovided at the output of the ultra-stable voltage amplifier 180 andprovided to the tuning unit 8 has the same stability than the voltagesupplied by the ultra-stable DC voltage source 4. Otherwise theperformance of the fourth embodiment of the inventive voltage supply 2may be somewhat reduced. But nevertheless the fourth embodiment of theinventive voltage supply 2 will provide a reference voltage of highaccuracy and high stability.

The average of the output voltage of the tuning unit divider 8 of thefourth embodiment of the inventive voltage supply 2 is after the tuningperiod equal to the average of the voltage supplied by the accurate DCvoltage source 6 during the tuning period. So the provided referencevoltage has—apart from a difference smaller the defined minimum value oftotal difference of the output voltage of the tuning 8 to the voltage ofthe accurate DC voltage source 6, below which the tuning by the controlunit 18 will be stopped—the value of the voltage supplied by theaccurate DC voltage source 6 and the same accuracy. But due to thefourth embodiment of the inventive power supply 2 the provided referencevoltage has now the stability of the ultra-stable DC voltage source 4.

FIG. 5 shows a fifth embodiment of the inventive power supply 2.

In this embodiment, in which in general all components are the same asin the first embodiment, the output voltage of the tunable voltagedivider 8 is provided to an amplifier 183—named in the furtherspecification output voltage amplifier 183. The output voltage amplifier183 amplifies the output voltage of the tunable voltage divider 8. So anamplified output voltage is provided at the output of the output voltageamplifier 183. This amplified output voltage is an example of a voltagebased on the output voltage of the tuning unit 8.

In the fifth embodiment of the inventive voltage supply 2 the amplifiedoutput voltage provided at the output of the output voltage amplifier183 shown by the arrow 184 and the voltage of the accurate voltagesource 6 shown by the arrow 16 are provided to the comparator 12 of theinventive power supply 2, which compares the voltage signals. Thecomparator 12 provides an output signal which is resulting from thecomparison of these two input voltages. The output signal can be asignal which is equal or proportional to the difference of the comparedvoltages. The output signal which is resulting from the comparison ofthese two input voltages can be also a digital signal which has only theoutput signals 0 and 1. By these signals it is only indicated which ofthe two input voltages has the higher value or higher absolute value.

In the fifth embodiment of the inventive voltage supply 2 the voltagesupplied by the ultra-stable DC voltage source 4 can have a lowerabsolute value than the voltage supplied by the accurate DC voltagesource 6.

The signal provided by the comparator 12, which results in the fifthembodiment of the inventive voltage supply 2 from the comparison of theamplified output voltage provided at the output of the output voltageamplifier 183 and the voltage of the accurate voltage source 6, isprovided to the control unit 18 as input signal shown by arrow 20.

The control unit 18 in the fifth embodiment of the inventive voltagesupply 2 is tuning the tuning unit 8 according to the signal provided bythe comparator 12 to minimise the difference between the amplifiedoutput voltage provided at the output of the output voltage amplifier183 and the voltage of the accurate DC voltage source 6. To achievethis, the control unit 18 provides an output signal, which is providedto the tuning unit 8 shown by arrow 22.

When the signal provided from the comparator 12 indicates, that thetotal difference of the amplified output voltage provided at the outputof the output voltage amplifier 183 to the voltage of the accurate DCvoltage source 6 is below a defined minimum value, the tuning by thecontrol unit 18 will be stopped. Preferably the ratio of the definedminimum value, when the tuning of the tunable voltage divider 8 isstopped, to the nominal average value of the voltage supplied by theaccurate DC voltage source 6 is below 500 ppm, preferably below 200 ppm,more preferably below 50 ppm and most preferably below 10 ppm.

For example in the fifth embodiment of the inventive voltage supply 2the defined minimum value can be 210 μV. For a nominal voltage theaccurate DC voltage source 6 of 7 V then the ratio of the definedminimum value to the nominal average value of the voltage supplied bythe accurate DC voltage source 6 is 30 ppm.

If the output signal of the comparator 12 is a digital signal whichindicates only, which of the two components has a higher value or ahigher absolute value and a output signal 22 provided by the controlunit 18 to the tuning unit 8 is inducing a change of the amplifiedoutput voltage provided at the output of the output voltage amplifier183 only for a value which is below the defined minimum value of thetotal difference of the amplified output voltage provided at the outputof the output voltage amplifier 183 to the voltage of the accurate DCvoltage source 6, when the tuning process shall be stopped, a change ofthe digital signal provided by the comparator due to the induced changeindicates that the total difference of the amplified output voltageprovided at the output of the output voltage amplifier 183 to thevoltage of the accurate DC voltage source 6 is below the defined minimumvalue. Accordingly the tuning process is stopped by the control unit 18.

Then the tuning unit 8 is providing an output voltage, which is in thefifth embodiment of the inventive voltage supply 2 a voltage having thestability of the ultra-stable DC voltage source 4 and the accuracy ofthe accurate DC voltage source 6. The average of the amplified outputvoltage provided at the output of the output voltage amplifier 183 isequal to the average of the voltage supplied by the accurate DC voltagesource 6. So the provided reference voltage, which is in the fifthembodiment the amplified output voltage provided at the output of theoutput voltage amplifier 183 has—apart from a difference smaller thedefined minimum value—the value of the voltage supplied by the accurateDC voltage source 6 and the same accuracy. But due to the inventivepower supply 2 it has now the stability of the ultra-stable DC voltagesource 4.

So the reference voltage provided by the fifth embodiment of theinventive power supply which is the amplified output voltage provided atthe output of the output voltage amplifier 183 is a voltage based on theoutput voltage of the tuning unit. In this embodiment the providedreference voltage is an amplified output voltage of the tuning unit 8,wherein the gain of the amplification is defined by the amplifyingoutput voltage amplifier 183.

If the signal provided by the comparator 12 which is provided to thecontrol unit 18 is equal or proportional to the difference of thecompared voltages, according to the detected voltage difference thecontrol unit 18 reacts and provides a signal to the tuning unit 8 toincrease or decrease its output voltage according to the detectedvoltage difference. In the fifth embodiment of the inventive voltagesupply 2 the absolute difference of the amplified output voltageprovided at the output of the output voltage amplifier 183 and thevoltage of the accurate DC voltage source 6 is reduced.

The output signal provided by the control unit 18 (arrow 22) in thefifth embodiment of the inventive voltage supply 2 is increasing theoutput voltage of the tunable voltage divider 8 and accordingly theamplified output voltage provided at the output of the output voltageamplifier 183 if the value of the voltage of the DC accurate voltagesource 6 is higher than the amplified output voltage provided at theoutput of the output voltage amplifier 183.

In particular if the comparator 12 in the fifth embodiment of theinventive voltage supply 2 is providing a digital signal, then thedigital signal provided by the comparator 12 indicates by a first valuethat the voltage of the accurate DC voltage source 6 is higher thanamplified output voltage provided at the output of the output voltageamplifier 183. The control unit reacts on the first value of the digitalsignal provided by the comparator and provides a signal to the tuningunit 8 to increase its output voltage and accordingly the amplifiedoutput voltage provided at the output of the output voltage amplifier183.

The output signal provided by the control unit 18 in the fifthembodiment of the inventive voltage supply 2 is decreasing the outputvoltage of the tuning unit 8 and accordingly the amplified outputvoltage provided at the output of the output voltage amplifier 183 ifthe value of the voltage of the accurate DC voltage source 6 is lowerthan the amplified output voltage provided at the output of the outputvoltage amplifier 183.

In particular if the comparator 12 in the fifth embodiment of theinventive voltage supply 2 is providing a digital signal, then thedigital signal provided by the comparator 12 indicates by a second valuethat the voltage of the accurate DC voltage source 6 is lower than theamplified output voltage provided at the output of the output voltageamplifier 183. The control unit reacts on the second value of thedigital signal provided by the comparator 12 and provides a signal tothe tuning unit 8 to decrease its output voltage and accordingly theamplified output voltage provided at the output of the output voltageamplifier 183.

Preferably the increase and decrease of the output voltage of the tuningunit 8 and accordingly the amplified output voltage provided at theoutput of the output voltage amplifier 183 is reduced stepwise by thecontrol unit 18 providing accordingly a signal to the tuning unit 8.

By this reduced change of the output voltage of the tuning unit 8 andaccordingly the reduced change of the amplified output voltage providedat the output of the output voltage amplifier 183 in the fifthembodiment of the inventive voltage supply 2 the absolute differencebetween amplified output voltage provided at the output of the outputvoltage amplifier 183 and the voltage of the accurate DC voltage source6 is minimised.

In the fifth embodiment of the FIG. 5 the amplified output voltageprovided at the output of the output voltage amplifier 183 of theinventive voltage supply 2 is applied as reference voltage at a voltageamplifier 24 shown by arrow 185, which is applying an amplified voltageto an electrode 28 shown by arrow 30. The amplified output voltageprovided at the output of the output voltage amplifier 183 is applied tothe voltage amplifier 24 via a switch 32. Accordingly the amplifiedoutput voltage provided at the output of the output voltage amplifier183 is applied at the switch 32 shown by arrow 185.

The switch 32 will connect in the fifth embodiment of the inventivevoltage supply 2 the amplified output voltage provided at the output ofthe output voltage amplifier 183 at a defined time after the voltagesupply 2 has been activated. Preferably the time delay to connect theamplified output voltage provided at the output of the output voltageamplifier 183 with the voltage amplifier 24 is chosen in that way thatthe tuning of the tuning unit 8 by the control unit 18 has been finishedbefore the amplified output voltage provided at the output of the outputvoltage amplifier 183 is connected with the voltage amplifier 24. Thenthe amplified output voltage provided at the output of the outputvoltage amplifier 183 is a reference voltage, having the accuracy of theaccurate DC voltage source 6 and stability of the ultra-stable DCvoltage source 4, which is applied to the voltage amplifier 24. Then thevoltage amplifier 24 is supplying a voltage of high accuracy andstability based on the reference voltage provided by the power supply 2to the electrode 28. This is in particularly correct, if the outputvoltage amplifier 183 has no influence on the stability of the amplifiedvoltage applied to the comparator 12, so that the amplified voltageprovided at the output of the output voltage amplifier 183 has the samestability than the voltage supplied by the ultra-stable DC voltagesource 4. Otherwise the performance of the fifth embodiment of theinventive voltage supply 2 may be somewhat reduced. But nevertheless thefifth embodiment of the inventive voltage supply 2 will provide areference voltage of high accuracy and high stability.

FIG. 6 shows a sixth embodiment of the inventive power supply 2.

In this embodiment, in which in general all components are the same asin the first embodiment, the output voltage of the tunable voltagedivider 8 is provided to an amplifier 186—named in the furtherspecification reference voltage pre-amplifier 186. The reference voltagepre-amplifier 186 amplifies the output voltage of the tunable voltagedivider 8. So an amplified output voltage is provided at the output ofthe reference voltage pre-amplifier 186. This amplified output voltageis a voltage based on the output voltage of the tuning unit 8 and isprovided as the reference voltage of the inventive voltage supply 2.

In the sixth embodiment of the inventive voltage supply 2 the outputvoltage of the tunable voltage divider 8 shown by the arrow 14 and thevoltage of the accurate voltage source 6 shown by the arrow 16 areprovided to the comparator 12 of the inventive power supply 2, whichcompares the voltage signals. The comparator 12 provides an outputsignal which is resulting from the comparison of these two inputvoltages. The output signal can be a signal which is equal orproportional to the difference of the compared voltages. The outputsignal which is resulting from the comparison of these two inputvoltages can be also a digital signal which has only the output signals0 and 1. By these signals it is only indicated which of the two inputvoltages has the higher value.

In the sixth embodiment of the inventive voltage supply 2 the voltagesupplied by the ultra-stable DC source 4 has a higher absolute valuethan the voltage supplied by the accurate DC voltage source 6.

Typically in the sixth embodiment of the inventive voltage supply 2 theabsolute value of the voltage of the ultra-stable DC source 4 is atleast 2% higher than the absolute value of the voltage of the accurateDC voltage source 6. Preferably the absolute value of the voltage of theultra-stable DC source 4 is at least 10% higher than the absolute valueof the voltage of the accurate DC voltage source 6. More preferably theabsolute value of the voltage of the ultra-stable DC source 4 is atleast 25% higher than the absolute value of the voltage of the accurateDC voltage source 6.

Typically the absolute value of the voltage of the ultra-stable DCsource 4 is not more than 500% higher than the absolute value of thevoltage of the accurate DC voltage source 6. Preferably the absolutevalue of the voltage of the ultra-stable DC source 4 is not more than200% higher than the absolute value of the voltage of the accurate DCvoltage source 6. More preferably the absolute value of the voltage ofthe ultra-stable DC source 4 is not more than 100% higher than theabsolute value of the voltage of the accurate DC voltage source 6.

For example in the sixth embodiment of the inventive voltage supply 2 anultra-stable DC source 4 can be used, which provide an average voltageof 9 V and an accurate DC voltage source 6 which provide a nominalvoltage of 7 V.

The signal provided by the comparator 12, which results in the sixthembodiment of the inventive voltage supply 2 from the comparison of theoutput voltage of the tuning unit 8 and the voltage of the accuratevoltage source 6 is provided to the control unit 18 as input signalshown by arrow 20.

The control unit 18 in the sixth embodiment of the inventive voltagesupply 2 is tuning the tuning unit 8 according to the signal provided bythe comparator 12 to minimise the absolute difference between the outputvoltage of the tuning unit 8 and the voltage of the accurate DC voltagesource 6. To achieve this, the control unit 18 provides an outputsignal, which is provided to the tuning unit 8 shown by arrow 22.

When the signal provided from the comparator 12 indicates, that thetotal difference of the output voltage of the tuning 8 to the voltage ofthe accurate DC voltage source 6 is below a defined minimum value, thetuning by the control unit 18 will be stopped. Preferably the ratio ofthe defined minimum value, when the tuning of the tunable voltagedivider 8 is stopped, to the nominal average value of the voltagesupplied by the accurate DC voltage source 6 is below 500 ppm,preferably below 200 ppm, more preferably below 50 ppm and mostpreferably below 10 ppm.

For example in the sixth embodiment of the inventive voltage supply 2the defined minimum value can be 140 μV. For a nominal voltage theaccurate DC voltage source 6 of 7 V then the ratio of the definedminimum value to the nominal average value of the voltage supplied bythe accurate DC voltage source 6 is 20 ppm.

Then the tuning unit 8 is providing an output voltage, which is in thesixth embodiment of the inventive voltage supply 2 provided to thereference voltage pre-amplifier 186 which amplifies the output voltageof the tunable voltage divider 8. Then the amplified voltage provided bythe reference voltage pre-amplifier 186 is the reference voltage of theinventive voltage supply 2 having the stability of the ultra-stable DCvoltage source 4 and the accuracy of the accurate DC voltage source 6.The average of the output voltage of the tuning unit divider 8 is equalto the average of the voltage supplied by the accurate DC voltage source6. So the provided voltage provided to the reference voltagepre-amplifier 186 has—apart from a difference smaller the definedminimum value—the value of the voltage supplied by the accurate DCvoltage source 6 and the same accuracy. Due to the inventive powersupply 2 the amplified output voltage provided at the output of thereference voltage pre-amplifier 186 as reference voltage has now thestability of the ultra-stable DC voltage source 4.

Preferably the increase and decrease of the output voltage of the tuningunit 8 is reduced stepwise by the control unit 18 providing accordinglya signal to the tuning unit 8.

By this reduced change of the output voltage of the tuning unit 8 in thesixth embodiment of the inventive voltage supply 2 the absolutedifference between the output voltage of the tuning unit 8 and thevoltage of the accurate DC voltage source 6 is minimised.

In the sixth embodiment of the FIG. 6 the amplified output voltageprovided at the output of the reference voltage pre-amplifier 186 isapplied as reference voltage at a voltage amplifier 24 shown by arrow187, which is applying an amplified voltage to an electrode 28 shown byarrow 30. The amplified output voltage provided at the output of thereference voltage pre-amplifier 186 is applied to the voltage amplifier24 via a switch 32. Accordingly the amplified output voltage provided atthe output of the reference voltage pre-amplifier 186 is applied at theswitch 32 shown by arrow 187.

The switch 32 will connect in the sixth embodiment of the inventivevoltage supply 2 the amplified output voltage provided at the output ofthe reference voltage pre-amplifier 186 with the voltage amplifier 24 ata defined time after the voltage supply 2 has been activated. Preferablythe time delay to connect the amplified output voltage provided at theoutput of the reference voltage pre-amplifier 186 with the voltageamplifier 24 is chosen in that way that the tuning of the tuning unit 8by the control unit 18 has been finished before the amplified outputvoltage provided at the output of the reference voltage pre-amplifier186 is connected with the voltage amplifier 24. Then amplified outputvoltage provided at the output of the reference voltage pre-amplifier186 is a reference voltage, having the accuracy of the accurate DCvoltage source 6 and stability of the ultra-stable DC voltage source 4,which is applied to the voltage amplifier 24. Then the voltage amplifier24 is supplying a voltage of high accuracy and stability based on thereference voltage provided by the power supply 2 to the electrode 28.This is in particularly correct, if the reference voltage pre-amplifier186 has no influence on the stability of the amplified voltage providedas reference voltage, so that the amplified voltage provided at theoutput of the reference voltage pre-amplifier 186 has the same stabilitythan the voltage supplied by the ultra-stable DC voltage source 4.Otherwise the performance of the sixth embodiment of the inventivevoltage supply 2 may be somewhat reduced. But nevertheless the sixthembodiment of the inventive voltage supply 2 will provide a referencevoltage of high accuracy and high stability.

FIG. 7 shows a seventh embodiment of the inventive power supply 2.

In this embodiment, in which in general all components are the same asin the first embodiment, the voltage supplied by the accurate DC voltagesource 6 is provided to an amplifier 188—named in the furtherspecification accurate voltage amplifier 188. The accurate voltageamplifier 188 amplifies the voltage supplied by the accurate DC voltagesource 6. So the amplified voltage is provided at the comparator 12shown by arrow 189. The amplified voltage at the output of the accuratevoltage amplifier 188, which is an accurate voltage, is a voltage basedon the supplied voltage of the accurate DC voltage source.

In the seventh embodiment of the inventive voltage supply 2 the outputvoltage of the tunable voltage divider 8 shown by the arrow 14 and theamplified voltage amplified by the accurate voltage amplifier 188 shownby the arrow 189 are provided to the comparator 12 of the inventivepower supply 2, which compares the voltage signals. The comparator 12provides an output signal which is resulting from the comparison ofthese two input voltages. The output signal can be a signal which isequal or proportional to the difference of the compared voltages. Theoutput signal which is resulting from the comparison of these two inputvoltages can be also a digital signal which has only the output signals0 and 1. By these signals it is only indicated which of the two inputvoltages has the higher value.

The signal provided by the comparator 12, which results in the seventhembodiment of the inventive voltage supply 2 from the comparison of theoutput voltage of the tuning unit 8 and the amplified voltage amplifiedby the accurate voltage amplifier 188 is provided to the control unit 18as input signal shown by arrow 20.

The control unit 18 in the seventh embodiment of the inventive voltagesupply 2 is tuning the tuning unit 8 according to the signal provided bythe comparator 12 to minimise the absolute difference between the outputvoltage of the tuning unit 8 and the amplified voltage amplified by theaccurate voltage amplifier 188. To achieve this, the control unit 18provides an output signal, which is provided to the tuning unit 8 shownby arrow 22.

When the signal provided from the comparator 12 indicates, that thetotal difference of the output voltage of the tuning 8 to the amplifiedvoltage amplified by the accurate voltage amplifier 188 is below adefined minimum value, the tuning by the control unit 18 will bestopped. Preferably the ratio of the defined minimum value, when thetuning of the tunable voltage divider 8 is stopped, to the nominalaverage value of the amplified voltage amplified by the accurate voltageamplifier 188 is below 500 ppm, preferably below 200 ppm, morepreferably below 50 ppm and most preferably below 10 ppm.

For example in the seventh embodiment of the inventive voltage supply 2the defined minimum value can be 280 μV. For a nominal voltage of theaccurate DC voltage source 6 of 3.5 V amplified by the accurate voltageamplifier 188 with a gain of 2 the ratio of the defined minimum value tothe nominal average value of the amplified voltage amplified by theaccurate voltage amplifier 188 is 40 ppm.

Then the tuning unit 8 is providing an output voltage, which is in theseventh embodiment of the inventive voltage supply 2 a reference voltagehaving the stability of the ultra-stable DC voltage source 4 and theaccuracy of the accurate DC voltage source 6. The average of the outputvoltage of the tuning unit divider 8 is equal to the average of theamplified voltage amplified by the accurate voltage amplifier 188. Sothe provided reference voltage has—apart from a difference smaller thedefined minimum value—the value of the amplified voltage amplified bythe accurate voltage amplifier 188 and the same accuracy. But due to theinventive power supply 2 it has now the stability of the ultra-stable DCvoltage source 4.

If the signal provided by the comparator 12 which is provided to thecontrol unit 18 is equal or proportional to the difference of thecompared voltages, according to the detected voltage difference thecontrol unit 18 reacts and provides a signal to the tuning unit 8 toincrease or decrease its output voltage according to the detectedvoltage difference. In the seventh embodiment of the inventive voltagesupply 2 the absolute difference of the output voltage of the tuningunit 8 and the amplified voltage amplified by the accurate voltageamplifier 188 is minimised.

The output signal provided by the control unit 18 (arrow 22) in theseventh embodiment of the inventive voltage supply 2 is increasing theoutput voltage of the tunable voltage divider 8 if the value of theamplified voltage amplified by the accurate voltage amplifier 188 ishigher than the output voltage of the tunable voltage divider 8.

In particular if the comparator 12 in the seventh embodiment of theinventive voltage supply 2 is providing a digital signal, then thedigital signal provided by the comparator 12 indicates by a first valuethat the amplified voltage amplified by the accurate voltage amplifier188 6 is higher than the output voltage of the tuning unit 8. Thecontrol unit reacts on the first value of the digital signal provided bythe comparator and provides a signal to the tuning unit 8 to increaseits output voltage.

The output signal provided by the control unit 18 in the seventhembodiment of the inventive voltage supply 2 is decreasing the outputvoltage of the tuning unit 8 if the amplified voltage amplified by theaccurate voltage amplifier 188 is lower than the output voltage of thetunable voltage divider 8.

In particular if the comparator 12 in the seventh embodiment of theinventive voltage supply 2 is providing a digital signal, then thedigital signal provided by the comparator 12 indicates by a second valuethat the amplified voltage amplified by the accurate voltage amplifier188 is lower than the output voltage of the tuning unit 8. The controlunit reacts on the second value of the digital signal provided by thecomparator 12 and provides a signal to the tuning unit 8 to decrease itsoutput voltage.

Preferably the increase and decrease of the output voltage of the tuningunit 8 is reduced stepwise by the control unit 18 providing accordinglya signal to the tuning unit 8.

By this reduced change of the output voltage of the tuning unit 8 in theseventh embodiment of the inventive voltage supply 2 the absolutedifference between the output voltage of the tuning unit 8 and theamplified voltage amplified by the accurate voltage amplifier 188 isminimised.

In the seventh embodiment of the FIG. 7 the output voltage of the tuningunit 8 of the inventive voltage supply 2 is applied as reference voltageat a voltage amplifier 24 shown by arrow 26, which is applying anamplified voltage to an electrode 28 shown by arrow 30. The outputvoltage of the tuning unit 8 is applied to the voltage amplifier 24 viaa switch 32. Accordingly the output voltage of the tuning unit 8 isdirectly applied at the switch 32 shown by arrow 34.

The switch 32 will connect in the seventh embodiment of the inventivevoltage supply 2 the output voltage of the tuning unit 8 with thevoltage amplifier 24 at a defined time after the voltage supply 2 hasbeen activated. Preferably the time delay to connect the output voltageof the tuning unit 8 with the voltage amplifier 24 is chosen in that waythat the tuning of the tuning unit 8 by the control unit 18 has beenfinished before the output voltage of the tuning unit 8 is connectedwith the voltage amplifier 24. Then the output voltage of the tuningunit 8 is a reference voltage, having the accuracy of the accurate DCvoltage source 6 and stability of the ultra-stable DC voltage source 4,which is applied to the voltage amplifier 24. Then the voltage amplifier24 is supplying a voltage of high accuracy and stability based on thereference voltage provided by the power supply 2 to the electrode 28.This is in particularly correct, if the accurate voltage amplifier 188has no influence on the accuracy of the reference voltage, so that theoutput voltage of the tuning unit 8 has the same accuracy than thevoltage supplied by the accurate DC voltage source 6.

Otherwise the performance of the seventh embodiment of the inventivevoltage supply 2 may be somewhat reduced. But nevertheless the seventhembodiment of the inventive voltage supply 2 will provide a referencevoltage of high accuracy and high stability.

FIG. 8 shows an eighth embodiment of the inventive power supply 2.

In this embodiment, in which in general all components are the same asin the first embodiment, only another embodiment of the tuning unit 8 isprovided. In this embodiment the tuning unit 8 of the voltage supply 2comprises at least one resistor 161 and a tunable voltage divider 162,which are connected in series.

In the eighth embodiment of the inventive voltage supply 2 the voltagesupplied by the DC ultra-stable voltage source 4 is applied to two(input) connectors of the tuning unit 8, wherein one connector isconnected with the at least one resistor 161 and the other with thetunable voltage divider 162.

The tuning unit 8 comprises an output connector (not shown) to which thetuning unit 8 provides an output voltage which is then provided to thecomparator shown by the arrow 14. The output voltage is provided bytaping the voltage from the resistor or the resistor network of thetunable voltage divider 162. Only a portion of the voltage supplied bythe ultra-stable DC voltage source 4 is applied to the tunable voltagedivider 162 and only this portion of the supplied voltage is tunable bythe tunable voltage divider 162 to provide an output voltage to the(output) connector. Accordingly only the portion of the voltage suppliedby the ultra-stable DC voltage source 4 to the tunable voltage divider162 can be used to adapt the output voltage of the tuning unit 8 to thevoltage supplied by the accurate DC voltage source 6. Typically theadaptable portion of the voltage supplied by the ultra-stable DC voltagesource 4 is higher than 10% of the voltage, preferably higher than 15%of the voltage and more preferably higher than 20% of the voltage.Typically the adaptable portion of the voltage supplied by theultra-stable voltage DC source 4 is below 50% of the voltage, preferablybelow 40% of the voltage and more preferably below 30 of the voltage.

FIG. 9 shows a ninth embodiment of the inventive power supply 2.

In this embodiment, in which in general all shown components are thesame as in the third embodiment, only another embodiment of the tuningunit 8 is provided. Another difference is, that in the ninth embodimentonly the output voltage of the tuning unit 8 is applied the switch 32.

In this embodiment the tuning unit 8 of the voltage supply 2 comprises afirst resistor 167 (R2) and a digital to analogue converter 164 (DAC),which are connected in parallel and a current to voltage converter,which is a transimpedance amplifier. The transimpedance amplifiercomprises an operational amplifier 165 and a feedback resistor 166 (R1).The digital to analogue converter 164 (DAC) is connected in series witha second resistor 163 (R3). The voltage supplied by the ultra-stablevoltage source 4 to the tuning unit 8 is applied at the parallelconnected digital to analogue converter 164 (DAC) and the first resistor167 (R2). The other end of this parallel connection is connected withthe input of the transimpedance amplifier, which provides at its outputthe output voltage of the tuning unit 8, which is provided to thecomparator 12 shown by the arrow 14 and directly applied to the switch32 shown by arrow 34. This voltage has the reverse polarity as thevoltage supplied by the ultra-stable voltage source 4. But the outputvoltage of the tuning unit 8 and the voltage supplied by the accuratevoltage source 6 have in this embodiment the same polarity.

In the ninth embodiment of the inventive voltage supply 2 the outputvoltage of the tunable voltage divider 8 shown by the arrow 14 and thevoltage of the accurate voltage source 6 shown by the arrow 16 areprovided to the comparator 12 of the inventive power supply 2, whichcompares the voltage signals. The comparator 12 provides an outputsignal which is resulting from the comparison of these two inputvoltages having the same polarity.

The value of the output voltage of the tuning unit 8 depends on theresistance values of the first resistor 167 (R2), the second resistor163 (R3), the feedback resistor 166 (R1) and the output voltage of thedigital to analogue converter 164 (DAC) as known by a skilled person.

The inventive voltage supply 2 comprises also a control unit, which is aprocessor 118. A computer program can be executed by the processor 118to operate the voltage supply 2 in accordance with the describedmethods.

The signal provided by the comparator 12, which results from thecomparison of the output voltage of the control unit 8 and the voltageof the accurate voltage source 6, is provided to the processor 118 asinput signal shown by arrow 20.

The processor 118 is tuning the digital to analogue converter (DAC) 164of the tuning unit 8 according to the digital signal provided by thecomparator 12 to minimise the absolute difference between the outputvoltage of control unit 8 and the voltage of the accurate DC voltagesource 6. To achieve this, the processor 118 provides a digital outputsignal, which is provided to the digital to analogue converter (DAC) 164shown by arrow 22.

The output signal provided by the processor 118 (arrow 22) is increasingthe absolute value of the output voltage of the digital to analogueconverter (DAC) 164 and accordingly the absolute value of the outputvoltage of the tuning unit 8 if the absolute value of the voltage of theaccurate DC voltage source 6 is higher than the absolute value of theoutput voltage of the tuning unit 8.

When the digital signal provided by the comparator 12 indicates by afirst value that the absolute value of the voltage supplied by theaccurate DC voltage source 6 is higher than the absolute value of theoutput voltage of the tuning unit 8, the processor 118 reacts on thefirst value of the digital signal provided by the comparator 12 andprovides a digital signal to the digital to analogue converter (DAC) 164to increase the absolute value of its output voltage.

The output signal provided by processor 118 is decreasing the absolutevalue of the output voltage of the digital to analogue converter (DAC)164 if the absolute value of the voltage of the accurate DC voltagesource 6 is lower than the absolute value of the output voltage of thetuning unit 8.

When the digital signal provided by the comparator 12 indicates by asecond value that the absolute value of the voltage supplied by theaccurate DC voltage source 6 is lower than absolute value of the outputvoltage of the digital to analogue converter (DAC) 108, the processor118 reacts on the second value of the digital signal provided by thecomparator 12 and provides a digital signal to the digital to analogueconverter (DAC) 164 to decrease the absolute value of its outputvoltage.

Preferably the increase and decrease of the output voltage of the tuningunit 8 is reduced stepwise by the processor 118 providing accordingly asignal to the digital to analogue converter (DAC) 164. By this reducedchange of the output voltage of the tuning unit 8 the absolutedifference between the output voltage of the tuning unit 8 and thevoltage of the accurate DC voltage source 6 is minimised.

Then the tuning unit 8 is providing an output voltage, which is areference voltage having the stability of the ultra-stable DC voltagesource 4 and the accuracy of the accurate DC voltage source 6. Theaverage of the output voltage of the tuning unit 8 is equal to theaverage of the voltage supplied by the accurate DC voltage source 6. Sothe provided reference voltage has—apart from a small difference—thevalue of the voltage supplied by the accurate DC voltage source 6 andthe same accuracy. But due to the inventive power supply 2 it has nowthe stability of the ultra-stable DC voltage source 4. This is inparticularly correct, if the components of the tuning unit 8 have noinfluence on the stability on output voltage of the tuning unit 8.Otherwise the performance of the ninth embodiment of the inventivevoltage supply 2 may be somewhat reduced. But nevertheless the ninthembodiment of the inventive voltage supply 2 will provide a referencevoltage of high accuracy and high stability.

In a preferred embodiment the digital to analogue converter 164comprises a resistor ladder network, in particular an R-2R resistorladder network. The digital signal provided by the processor 118 in thispreferred embodiment is a digital signal of a specific number of bits.Preferably the digital signal is a signal of at least 16 bits,particular preferably of at least 20 bits. Based on the digital signalprovided by the comparator 12 the processor 118 is applying the methodof successive approximation to minimise the absolute difference betweenthe output voltage of tuning unit 8 and the voltage of the accurate DCvoltage source 6 as explained before in detail. In this method accordingto the bits of the digital signal provided by the processor 118 to thedigital to analogue converter (DAC) 164 the output voltage of thedigital to analogue converter (DAC) 164 is determined by the R-2Rresistor ladder network. The resistor network is dividing with each setbit (value 1) the voltage applied at the digital to analogue converter(DAC) 164 into equal shares. So the first bit is dividing the voltage tothe share ½, the second is dividing the remaining voltage into shares of¼, the third is dividing the remaining voltage into shares of ⅛ and soforth. Accordingly to the set bits provided by the processor 118 theoutput voltage of the digital to analogue converter (DAC) 164 isprovided.

During the iteration steps of the method each of the bits of the digitalsignal provided by the processor 118 to the digital to analogueconverter (DAC) 164 is set and the output voltage of the tuning unit 8is correctly adjusted to the voltage of the accurate DC voltage source6. The accuracy of this adjustment is increasing fast.

In the embodiment of the FIG. 9 the output voltage of the tuning unit 8of the inventive voltage supply 2 is applied as reference voltage at avoltage amplifier 24 shown by arrow 26, which is applying an amplifiedvoltage to an electrode 28 shown by arrow 30. The output voltage of thetuning unit 8 is applied at the voltage amplifier 24 via a switch 32after the tuning process has been finished. Accordingly the outputvoltage of the tuning unit 8 is directly applied to the switch 32 shownby arrow 34.

FIG. 10 shows a tenth embodiment of the inventive power supply 2.

In this embodiment, in which in general all shown components are thesame as in the third embodiment, only another embodiment of the tuningunit 8 is provided. Another difference is, that in the tenth embodimentonly the output voltage of the tuning unit 8 is applied the switch 32.

In this embodiment the tuning unit 8 of the voltage supply 2 comprises afirst resistor 171 (R2) and a digital to analogue converter 172 (DAC)and a current to voltage converter which is a transimpedance amplifier.The transimpedance amplifier comprises an operational amplifier 173 anda feedback resistor 174 (R1).

At the first resistor 171 (R2) the voltage provided by the ultra-stableDC voltage source 4 is applied and at the digital to analogue converter172 (DAC) the voltage provided by the accurate DC voltage source 6 isapplied. The digital to analogue converter 172 (DAC) is connected inseries with a second resistor 175 (R3).

The first resistor 171 (R2) and the digital to analogue converter 172(DAC) are connected at a node, which is connected with the input of thetransimpedance amplifier, which provides its output to an input of aninverting amplifier 199. At the output of the inverting amplifier 199 isprovided the output voltage of the tuning unit 8. This voltage isprovided to the comparator 12 shown by the arrow 14 and directly appliedto the switch 32 shown by arrow 34.

In the tenth embodiment of the inventive voltage supply 2 the outputvoltage of the tuning unit 8 shown by the arrow 14 and the voltage ofthe accurate voltage source 6 shown by the arrow 16 are provided to thecomparator 12 of the inventive power supply 2, which compares thevoltage signals. The comparator 12 provides an output signal which isresulting from the comparison of these two input voltages.

The value of the output voltage of the tuning unit 8 depends on theresistance values of the first resistor 171 (R3), the second resistor175 (R2), the feedback resistor 174 (R1) and the output voltage of thedigital to analogue converter 172 (DAC) as known by a skilled person.The output voltage of the digital to analogue converter 172 (DAC) isrelated to the voltage supplied by the accurate DC voltage source 6 tothe digital to analogue converter 172 (DAC).

The inventive voltage supply 2 comprises also a control unit, which is aprocessor 118. A computer program can be executed by the processor tooperate the voltage supply 2 in accordance with the described methods.

The signal provided by the comparator 12, which results from thecomparison of the output voltage of the control unit 8 and the voltageof the accurate voltage source 6, is provided to the processor 118 asinput signal shown by arrow 20.

The processor 118 is tuning the digital to analogue converter (DAC) 172of the tuning unit 8 according to the digital signal provided by thecomparator 12 to minimise the absolute difference between the outputvoltage of control unit 8 and the voltage of the accurate DC voltagesource 6. To achieve this, the processor 118 provides a digital outputsignal, which is provided to the digital to analogue converter (DAC) 172shown by arrow 22.

The output signal provided by the processor 118 (arrow 22) is increasingthe absolute value of the output voltage of the digital to analogueconverter (DAC) 172 and accordingly the absolute value of the outputvoltage of the tuning unit 8 if the absolute value of the voltage of theaccurate DC voltage source 6 is higher than absolute value of the outputvoltage of the tuning unit 8.

When the digital signal provided by the comparator 12 indicates by afirst value that the absolute value of the voltage supplied by theaccurate DC voltage source 6 is higher than the absolute value of theoutput voltage of the tuning unit 8, the processor 118 reacts on thefirst value of the digital signal provided by the comparator 12 andprovides a digital signal to the digital to analogue converter (DAC) 172to increase the absolute value of its output voltage.

The output signal provided by the processor 118 is decreasing theabsolute value of the output voltage of the digital to analogueconverter (DAC) 172 if the absolute value of the voltage of the accurateDC voltage source 6 is lower than the absolute value of the outputvoltage of the tuning unit 8.

When the digital signal provided by the comparator 12 indicates by asecond value that the absolute value of the voltage supplied by theaccurate DC voltage source 6 is lower than the absolute value of the ofthe output voltage of the tuning unit 8, the processor 118 reacts on thesecond value of the digital signal provided by the comparator 12 andprovides a digital signal to the digital to analogue converter (DAC) 172to decrease the absolute value of its output voltage.

Preferably the increase and decrease of the output voltage of the tuningunit 8 is reduced stepwise by the processor 118 providing accordingly asignal to the digital to analogue converter (DAC) 172. By this reducedchange of the output voltage of the tuning unit 8 the absolutedifference between the output voltage of the tuning unit 8 and thevoltage of the accurate DC voltage source 6 is minimised.

Then the tuning unit 8 is providing an output voltage, which is areference voltage, having the stability of the ultra-stable DC voltagesource 4 and the accuracy of the accurate DC voltage source 6. Theaverage of the output voltage of the tuning unit 8 is equal to theaverage of the voltage supplied by the accurate DC voltage source 6. Sothe provided reference voltage has—apart from a small difference—thevalue of the voltage supplied by the accurate DC voltage source 6 andthe same accuracy. But due to the inventive power supply 2 it has nowthe stability of the ultra-stable DC voltage source 4. This is inparticularly correct, if the components of the tuning unit 8 have noinfluence on the stability on output voltage of the tuning unit 8.

Otherwise the performance of the tenth embodiment of the inventivevoltage supply 2 may be somewhat reduced. But nevertheless the tenthembodiment of the inventive voltage supply 2 will provide a referencevoltage of high accuracy and high stability.

In a preferred embodiment the digital to analogue converter 172comprises a resistor ladder network, in particular an R-2R resistorladder network, which can be used in the same way as described for theninth embodiment.

During the iteration steps of the proposed method each of the bits ofthe digital signal provided by the processor 118 to the digital toanalogue converter (DAC) 172 is set and the output voltage of the tuningunit 8 is correctly adjusted to the voltage of the accurate DC voltagesource 6. The accuracy of this adjustment is increasing fast becausewith each set of a further bit the adjustment of the output voltage ofthe digital to analogue converter (DAC) 172 is bisected which isprovoking the adjustment of the output voltage of the tuning unit 8.

In the embodiment of the FIG. 10 the output voltage of the tuning unit 8of the inventive voltage supply 2 is applied as reference voltage at avoltage amplifier 24 shown by arrow 26, which is applying an amplifiedvoltage to an electrode 28 shown by arrow 30. The output voltage of thetuning unit 8 is applied at the voltage amplifier 24 via a switch 32after the tuning process has been finished. Accordingly the outputvoltage of the tuning unit 8 is directly applied to the switch 32 shownby arrow 34.

FIG. 11 shows an eleventh embodiment of the inventive power supply 2.

In this embodiment, in which in general all components are the same asin the first embodiment, only another embodiment of the tuning unit 8 isprovided.

In this embodiment of the invention the tuning unit 8 of the voltagesupply comprises two tunable voltage dividers 191 and 192, which arepreferably two digital to analogue converters (DAC). The voltagesupplied by the ultra-stable DC voltage source 4 to the tuning unit 8 isapplied at a first tunable voltage divider 191 (T1) of the two tunablevoltage dividers, which provides an output voltage to an (output)connector. This output voltage is then applied at the second tunablevoltage divider 192 (T 2) of the two tunable voltage dividers, whichprovides an output voltage to an (output) connector, which is the outputvoltage of the tuning unit 8. The first tunable voltage divider 191 isused to tune the output voltage of the tuning unit 8 coarse and thesecond tunable voltage divider 192 is used to tune the output voltage ofthe tuning unit 8 in a fine manner.

The tuning unit 8 comprises an output connector (not shown) to which thetuning unit 8 provides an output voltage which is then provided to thecomparator 12 shown by the arrow 14. The output voltage is provided bytaping the voltage from a resistor or the resistor network of the secondtunable voltage divider 192.

In the eleventh embodiment of the inventive voltage supply 2 the voltagesupplied by the ultra-stable DC source 4 has a higher absolute valuethan the voltage supplied by the accurate DC voltage source 6.

Typically in the eleventh embodiment of the inventive voltage supply 2the absolute value of the voltage of the ultra-stable DC source 4 is atleast 2% higher than the absolute value of the voltage of the accurateDC voltage source 6. Preferably the absolute value of the voltage of theultra-stable DC source 4 is at least 10% higher than the absolute valueof the voltage of the accurate DC voltage source 6. More preferably theabsolute value of the voltage of the ultra-stable DC source 4 is atleast 25% higher than the absolute value of the voltage of the accurateDC voltage source 6.

Typically the absolute value of the voltage of the ultra-stable DCsource 4 is not more than 500% higher than the absolute value of thevoltage of the accurate DC voltage source 6. Preferably the absolutevalue of the voltage of the ultra-stable DC source 4 is not more than200% higher than the absolute value of the voltage of the accurate DCvoltage source 6. More preferably the absolute value of the voltage ofthe ultra-stable DC source 4 is not more than 100% higher than theabsolute value of the voltage of the accurate DC voltage source 6.

The first tunable voltage divider 191 to which the voltage of theultra-stable DC source 4 is applied is used to tune the output voltageof the tuning unit 8 coarse. So typically the output voltage of thetuning unit 8 is tuned by the first tunable voltage divider 191 with aselectivity of 1% to 5% of the voltage of the accurate DC voltage source6, preferably with a selectivity of 200 ppm to 1,000 ppm of the voltageof the accurate DC voltage source 6.

The second tunable voltage divider 192 is used to tune the outputvoltage of the tuning unit 8 in a fine manner, so that the totaldifference of the output voltage of the tuning 8 to the voltage of theaccurate DC voltage source 6 is below a defined minimum value, when thetuning by the control unit 18 will be stopped. Preferably due to thetuning of the second tunable voltage divider 192 the ratio of thedefined minimum value, when the tuning of the tunable voltage divider 8is stopped, to the nominal average value of the voltage supplied by theaccurate DC voltage source 6 is below 500 ppm, preferably below 200 ppm,more preferably below 50 ppm and most preferably below 10 ppm.

The control unit 18 in the eleventh embodiment of the inventive voltagesupply 2 is tuning the first tunable voltage divider 191 and the secondtunable voltage divider 192 of the tuning unit 8 according to the signalprovided by the comparator 12 to minimise the absolute differencebetween the output voltage of the tuning unit 8 and the voltage of theaccurate DC voltage source 6. To achieve this the control unit 18provides an two output signals, a first output signal for coarse tuningprovided to the first tunable voltage divider 191 of the tuning unit 8shown by arrow 193 and a second output signal for fine tuning providedto the second tunable voltage divider 192 of the tuning unit 8 shown byarrow 194.

The control unit 18 provides the two output signals based on the outputsignal shown by the arrow 20 of the comparator 12 in the same way asdescribed for the first embodiment. The difference is that the firstoutput signal shown by arrow 193 is provided by the control unit 18 onlyfor a coarse adjustment to the first tunable voltage divider 191 andthat the second output signal shown by arrow 194 is provided by thecontrol unit 18 to the second tunable voltage divider 192 to minimisethe total difference between the output voltage of the tuning unit 8 andthe voltage of the accurate DC voltage source 6 until the totaldifference is below a defined minimum value and the tuning process isstopped. Due to the use of two tunable voltage dividers the adjustmentcan be faster and is more sensitive to react on small total differencesbetween the output voltage of the tuning 8 and the voltage of theaccurate DC voltage source 6. At the beginning of the tuning process thecontrol unit 18 is providing the first output signal shown by arrow 193and may also provide the second output signal shown by arrow 194. Whenthe total difference between the output voltage of the tuning unit 8 andthe voltage of the accurate DC voltage source 6 is during the tuningprocess below the selectivity of the first tunable voltage divider 191,the control unit 18 is preferably only further adapting the secondoutput signal shown by arrow 194 provided to the second tunable voltagedivider 192 to minimise the total difference between the output voltageof the tuning unit 8 and the voltage of the accurate DC voltage source 6until the total difference is below a defined minimum value, wherein thefirst output signal shown by arrow 193 remains unchanged.

In the eleventh embodiment of the inventive voltage supply 2 the outputvoltage of the tuning unit 8 is providing a reference voltage, havingthe accuracy of the accurate DC voltage source 6 and stability of theultra-stable DC voltage source 4. This is in particularly correct, ifthe two tunable voltage dividers 191 and 192 have no influence on thestability of the reference voltage. Otherwise the performance of theeleventh embodiment of the inventive voltage supply 2 may be somewhatreduced. But nevertheless the eleventh embodiment of the inventivevoltage supply 2 will provide a reference voltage of high accuracy andhigh stability.

FIG. 12 shows a twelfth embodiment of the inventive power supply 2.

In this embodiment, in which in general all components are the same asin the ninth embodiment, only the tuning unit 8 comprises additionalcomponents.

In this embodiment of the invention the tuning unit 8 of the voltagesupply 2 comprises a second digital to analogue converters 196 (DAC2).The first resistor 167 (R2) and the two digital to analogue converters,the first digital to analogue converters 164 (DAC1) and the seconddigital to analogue converters 196 (DAC2) are provided in parallellines.

The first digital to analogue converter 164 (DAC1) is connected inseries with a second resistor 163 (R3) and the second digital toanalogue converter 196 (DAC2) is connected in series with a thirdresistor 197 (R4). The voltage supplied by the ultra-stable voltagesource 4 to the tuning unit is applied at the first digital to analogueconverter 164 (DAC1), the second digital to analogue converter 164(DAC2) and the first resistor 167 (R2). The other end of the parallellines, in which the first resistor 167 (R2) and the two digital toanalogue converters, the first digital to analogue converters 164 (DAC1)and the second digital to analogue converters 196 (DAC2) are provided,is connected with the input of the transimpedance amplifier, whichprovides at its output the output voltage of the tuning unit 8, which isprovided to the comparator 12 shown by the arrow 14 and directly appliedto the switch 32 shown by arrow 34. This voltage has the reversepolarity as the voltage supplied by the ultra-stable voltage source 4.But the output voltage of the tuning unit 8 has the same polarity as thevoltage supplied by the accurate voltage source 6.

In the twelfth embodiment of the inventive voltage supply 2 the outputvoltage of the tunable voltage divider 8 shown by the arrow 14 and thevoltage of the accurate voltage source 6 shown by the arrow 16 areprovided to the comparator 12 of the inventive power supply 2, whichcompares the voltage signals. The comparator 12 provides an outputsignal which is resulting from the comparison of these two inputvoltages having the same polarity.

The value of the output voltage of the tuning unit 8 depends on theresistance values of the first resistor 167 (R2), the second resistor163 (R3), the third resistor 197 (R4), the feedback resistor 166 (R1),the output voltage of the first digital to analogue converter 164 (DAC1)and the output voltage of the second digital to analogue converter 196(DAC2) as known by a skilled person.

The inventive voltage supply 2 comprises also a control unit, which isprocessor 118. A computer program can be executed by the processor tooperate the voltage supply 2 in accordance with the described methods.

The signal provided by the comparator 12, which results from thecomparison of the output voltage of the tuning unit 8 and the voltage ofthe accurate voltage source 6, is provided to the processor 118 as inputsignal shown by arrow 20.

The processor 118 is tuning the first digital to analogue converter(DAC1) 164 and second digital to analogue converter (DAC2) 196 of thetuning unit 8 according to the digital signal provided by the comparator12 to minimise the absolute difference between the output voltage oftuning unit 8 and the voltage of the accurate DC voltage source 6. Toachieve this, the processor 118 provides two digital output signals,which are provided to the first digital to analogue converter (DAC1) 164shown by arrow 22 and the second digital to analogue converter (DAC2)196 shown by arrow 198.

In this configuration the first digital to analogue converter 164 (DAC1)is provided for a coarse tuning of the output voltage of the tuning unit8 and the second digital to analogue converter 196 (DAC2) is providedfor a fine tuning of the output voltage of the tuning unit 8. The seconddigital to analogue converter 196 (DAC2) is connected in series with thethird resistor 197 (R4) of higher resistivity than the second resistor163 (R3) for the fine tuning of the tuning unit 8.

The control unit 18 provides the two output signals based on the outputsignal shown by the arrow 20 of the comparator 12 in the same way asdescribed for the ninth embodiment. The difference is that the firstoutput signal shown by arrow 22 is provided by the control unit 18 onlyfor a coarse adjustment to the first digital to analogue converter 164(DAC1) and that the second output signal shown by arrow 198 is providedby the control unit 18 to the second digital to analogue converter 196(DAC2) to minimise the total difference between the output voltage ofthe tuning unit 8 and the voltage of the accurate DC voltage source 6after the coarse adjustment to the first digital to analogue converter164 (DAC1) until the total difference is below a defined minimum valueand the tuning process is stopped.

In detail, at the beginning of the tuning process the control unit 18 isproviding the first output signal shown by arrow 22 for a coarseadjustment to the first digital to analogue converter 164 (DAC1) and mayalso provide the second output signal shown by arrow 198 to the seconddigital to analogue converter 196 (DAC2). When the total differencebetween the output voltage of the tuning unit 8 and the voltage of theaccurate DC voltage source 6 is during the tuning process below theselectivity of the first digital to analogue converter 164 (DAC1), thecontrol unit 18 is preferably only further adapting the second outputsignal shown by arrow 198 provided to the second digital to analogueconverter 196 (DAC2) to minimise the total difference between the outputvoltage of the tuning unit 8 and the voltage of the accurate DC voltagesource 6 until the total difference is below a defined minimum value,wherein the first output signal shown by arrow 22 remains unchanged.

Due to the use of two digital to analogue converters the adjustment canbe faster and more sensitive to react on small total differences betweenthe output voltage of the tuning unit 8 and the voltage of the accurateDC voltage source 6.

In the twelfth embodiment of the inventive voltage supply 2 the outputvoltage of the tuning unit 8 is providing a reference voltage, havingthe accuracy of the accurate DC voltage source 6 and stability of theultra-stable DC voltage source 4. This is in particularly correct, ifthe components of the tuning unit 8 divider have no influence on thestability of the reference voltage. Otherwise the performance of thetwelfth embodiment of the inventive voltage supply 2 may be somewhatreduced. But nevertheless the twelfth embodiment of the inventivevoltage supply 2 will provide a reference voltage of high accuracy andhigh stability.

In FIGS. 13 a-13 d details of the electrical circuit of the thirdembodiment of the inventive voltage supply are shown.

In FIG. 13 the ultra-stable DC voltage source 4 is shown in detail.Shown are the output connectors 120,122 of the ultra-stable DC voltagesource 4 providing the voltage to the digital to analogue converter(DAC) 108.

In FIG. 13 b the comparator 12 is shown in detail. The input connector130 is connected with the accurate DC voltage source 6 and the inputconnector 132 is connected with the output of the digital to analogueconverter (DAC) 108. The comparator 12 is providing via the outputconnectors 140 and 142 a digital signal to the processor 118.

In FIG. 13 c the digital to analogue converter (DAC) 108 is shown indetail. Via the input connectors 150, 152 the voltage of theultra-stable DC voltage source 4 is provided to the digital to analogueconverter (DAC) 108. The output connector 160 is providing the outputvoltage of the digital to analogue converter (DAC) 108, which is theultra-stable and accurate reference voltage provided by the voltagesupply 2. Via the input connector 170 the processor 118 provides itsinput signal to the digital to analogue converter (DAC) 108.

In FIG. 13 d the switch 32 of the voltage supply 2 is shown in detail.It is shown the accurate DC voltage source 6, which is connected withthe switch 32. Via the input connector 190 is the output voltage of thedigital to analogue converter (DAC) 108 provided to the switch. Furtherthe switch control signal is provided via the line 38 (shown in FIGS. 2and 3 as arrow 38). The switch is providing the voltage provided by thepower supply 2 via the output connector 195 to the amplifier 24.

In FIG. 14 is shown a measurement of the time stability behaviour of thereference voltage provided by the inventive voltage supply 2 of thethird embodiment of the invention over a time period of 24 hours. Shownis the relative absolute maximum deviation of the maximum value andminimum value in relation to the average voltage of the referencevoltage over the time by the dots. By the “+”-symbol is shown relativeabsolute maximum deviation of the maximum value and minimum value inrelation to the average voltage over the time of the accurate DC voltagesource 6 for comparison. Due to the inventive power supply the relativeabsolute maximum deviation of the reference voltage could be reducedbelow 1 ppm, even below 0.6 ppm, over the whole time. For the accurateDC voltage source 6 the relative absolute maximum deviation is muchhigher and varies over the investigated time period.

So by the inventive voltage supply 2 the time stability of the providedreference voltage is much increased in comparison to the voltage,provided only by the accurate DC voltage source 6.

In FIG. 15 is shown a measurement of the temperature behaviour of thereference voltage provided by the inventive voltage supply 2 of thethird embodiment of the invention over a temperature range between 56°C. and 64.5° C. Shown by the dots is the relative absolute deviation ofthe average voltage of the reference voltage at the differenttemperatures of the reference voltage of the inventive power supply 2.This relative absolute deviation of average voltage is shown in relationto the lowest average voltage of the reference voltage in thetemperature range having the relative deviation of 0. By the “+”-symbolis shown the relative absolute deviation of average voltage of thereference voltage provided by the accurate DC voltage source 6 at thedifferent temperatures in relation to the lowest average voltage of theaccurate DC voltage source 6 in the temperature range having therelative deviation of 0 for comparison. The measurements started at thetemperature of 56° C. Due to the inventive power supply the relativeabsolute maximum deviation of the reference voltage, which is therelative maximum deviation of the maximum value and the minimum value ofthe voltage in the specific temperature range, could be reduced below 1ppm over the whole temperature range. For the accurate DC voltage source6 the absolute relative maximum deviation is much higher and varies overthe investigated temperature range. Also a strong hysteresis effect isvisible for the accurate DC voltage source 6, when the temperature isincreased and decreased showing higher relative deviations forincreasing temperatures.

So by the inventive voltage supply 2 the temperature stability of theprovided reference voltage is much increased in comparison to thevoltage, provided only by the accurate DC voltage source 6.

The reference voltage provided by the inventive power supply 2 can beused to supplying a voltage to at least one electrode of a massspectrometer.

The electrode of a mass spectrometer can be in particular an electrodeof a time-of-flight mass spectrometer, in particular of amulti-reflection time-of-flight mass spectrometer in which the referencevoltage provided by the inventive voltage supply can be used to providea voltage to mirror electrodes, e.g. a specific electrode of the mirrorelectrodes or all electrodes of the mirror electrodes. In FIGS. 16 a and16 b a first embodiment of multi-reflection time-of-flight mass analyser200 is shown, which is used in a multi-reflection time-of-flight massspectrometer. The inventive power supply 2 can be used in suchmulti-reflection time-of-flight mass spectrometer. The reference voltageis provided to at least one voltage amplifier 24 (not shown) ofmulti-reflection time-of-flight mass spectrometer, which then provides avoltage to electrodes, e.g. at least one mirror electrode, ofmulti-reflection time-of-flight mass analyser 200, which has typically avalue in the range of kilovolts (kV).

In particular in FIGS. 16 a and 16 b the multi-reflection time-of-flightmass analyser 200 is shown schematically which is known by a skilledperson. The multi-reflection time-of-flight mass analyser 200 comprisesparallel ion-optical mirrors 210, 211 elongated linearly along a driftlength. FIG. 16 a shows the analyser in the X-Y plane and FIG. 16 bshows the same analyser in the X-Z plane. Opposing ion-optical mirrors210, 211 are elongated along a drift direction Y and are arrangedparallel to one another. Ions are injected from ion injector 213 withinjection angle Θ to axis X and angular divergence δθ, in the X-Y plane.Accordingly, three ion flight paths are depicted, 216, 217, 218. Theions travel into mirror 210 and are turned around to proceed out ofmirror 210 and towards mirror 211, whereupon they are reflected inmirror 211 and proceed back to mirror 210 following a zigzag ion flightpath, drifting relatively slowly in the drift direction Y. Aftermultiple reflections in mirrors 210, 211 the ions reach a detector 214,upon which they impinge, and are detected. In some embodiments ofanalysers the ion injector 213 and detector 214 are located outside thevolume bounded by the mirrors. FIG. 16 b is a schematic diagram of themulti-reflection time-of-flight mass analyser 200 of FIG. 16 a shown insection, i.e. in the X-Z plane, but with the ion flight paths 216, 217,218, ion injector 213 and detector 214 omitted for clarity.

Each ion-optical mirror 210, 211 comprises three elongate parallelmirror electrodes. The ion ion-optical mirror 210 comprises the threemirror electrodes 220, 221, 222 and the ion ion-optical mirror 211comprises the three mirror electrodes 230, 231, 232.

The ions in a multi-reflection time-of-flight mass spectrometer 200 arereflected between opposing ion-optical mirrors 210, 211 several timeswhile they are drifting along the drift direction Y. It is possible,that the ions are injected with a small injection angle Θ to axis X.Accordingly the number of the reflection of the ions and the length ofthe flight path of the ions will increase. Due to the multi-reflectionof the ions at the ion-optical mirrors 210, 211 comprising severalmirror electrodes 220, 221, 222, 230, 231, 232, each electrode 220, 221,222, 230, 231, 232 has preferably to be provided with an accurate andultra-stable voltage. Any instability could change the trajectory of theions which are oscillating between the reflecting ion-optical mirrors210, 211. Accordingly a changed flight-time of the analysed ions wouldresult in time-of-flight mass spectra of lower resolving power orchanging mass-to-charge-calibration of the detected time-of-flight massspectra.

It is also possible and may be sufficient, that only to a specificelectrode in each mirror electrode, e.g. the electrodes 220 and 230, anaccurate and ultra-stable voltage is provided.

Therefore, at least one inventive voltage supply 2, for example one ofthe twelve embodiments described before, is used in the multi-reflectiontime-of-flight mass spectrometer to provide an accurate and ultra-stablereference voltage. The reference voltage is provided to the voltageamplifiers 24 of multi-reflection time-of-flight mass spectrometer,which then provides a voltage to mirror electrodes 220, 221, 222, 230,231, 232 of ion-optical mirrors 210, 211 of the of multi-reflectiontime-of-flight mass analyser 200, which is typically in the range ofkilovolts (kV). Based on the specific concept of the multi-reflectiontime-of-flight mass analyser 200, to each of the mirror electrodes 220,221, 222, 230, 231, 232 of an ion-optical mirrors 210 is the samevoltage provided or more preferably to each of the mirror electrodes220, 221, 222 of an ion-optical mirrors 210 a different voltage isprovide by voltage amplifiers 24 which are using the accurate andultra-stable reference voltage of an inventive voltage supply 2. Onevoltage amplifier 24 can be used to apply the same voltage to more thanone electrode. In particular, typically to mirror electrodes having thesame function in both ion-optical mirrors 210, 211 the same voltage isapplied by one voltage amplifier 24, for example to the outer mirrorelectrodes 220, 230 of both ion-optical mirrors 210, 211. Similarly,another voltage amplifier 24 can supply the same voltage to mirrorelectrodes 221, 231 and a further voltage amplifier 24 can supply thesame voltage to mirror electrodes 222, 232. The voltage applied at themirror electrodes has typically an absolute value in the range of 1 kVup to 12 kV, preferably in the range of 2 KV up to 8 kV. In amulti-reflection time-of-flight mass spectrometer, for example of theshown embodiment can be used one inventive voltage supply 2 to providean accurate and ultra-stable reference voltage, but also more inventivevoltage supplies 2 to provide accurate and ultra-stable referencevoltages. E.g. to each voltage amplifier 24 can be assigned a separatevoltage supply 2 to provide an accurate and ultra-stable referencevoltage for each voltage amplifier 24.

In FIG. 17 a second embodiment of multi-reflection time-of-flight massanalyser 300 is shown, which is used in a multi-reflectiontime-of-flight mass spectrometer. The inventive power supply 2 can beused also in such multi-reflection time-of-flight mass spectrometer. Thereference voltage is provided to at least one voltage amplifier 24 (notshown) of the multi-reflection time-of-flight mass spectrometer, whichthen provides a voltage to electrodes of multi-reflection time-of-flightmass analyser 300, which is typically in the range of kilovolts (kV).The embodiment of the multi-reflection time-of-flight mass analyser 300is known by skilled persons and described in WO 2013/110587. Theinventive voltage supply 2 can be also used in other multi-reflectiontime-of-flight mass spectrometer described in WO 2013/110587 to providean accurate and ultra-stable reference voltage.

In particular, in FIG. 17 the multi-reflection time-of-flight massanalyser 300 is shown schematically. The multi-reflection time-of-flightmass analyser 300 comprises ion-optical mirrors 310, 311 elongatedlinearly along a drift length. FIG. 17 shows the analyser in the X-Yplane. Opposing ion-optical mirrors 310, 311 are elongated parabolicallyalong a drift direction Y. The multi-reflection time-of-flight massanalyser 300 is further comprising compensation electrodes 365-1, 366-1,367-1, 365-2, 366-2, 367-2. As a more technological implementation,parabolic shapes could be approximated by circular arcs (which could bethen made on a turning machine). Compensation electrodes 365-1, 366-1,367-1, 365-2, 366-2, 367-2 allow further advantages to be provided, inparticular that of reducing time-of-flight aberrations. The embodimentof FIG. 17 is similar to that of FIGS. 16 a and 16 b, and similarconsiderations apply to the general ion motion from the injector 363 tothe detector 364 the ions undergoing a plurality of oscillations 360between mirrors ion-optical mirrors 310, 311. Due to the parabolic shapeof the ion optical mirrors the ions are reflected back at high Y values(at the right side of the mass analyser), then moving in negative driftdirection Y and finally impinge on the detector which is arranged on thesame side as the injector of the ions 363. Three pairs of compensationelectrodes 365-1, 365-2 as one pair, 366-1, 366-2 as another pair and367-1, 367-2 as a further pair, comprise extended surfaces in the X-Yplane facing the ion beam, the electrodes being displaced in +/−Z fromthe ion beam flight path, i.e. each compensation electrode 365-1, 366-1,367-1, 365-2, 366-2, 367-2 has a surface substantially parallel to theX-Y plane located either side of a space extending between the opposingion-optical mirrors 310, 311. In use, the compensation electrodes365-1,365-2 are electrically biased, both electrodes having voltageoffset U(Y)>0 applied in case of positive ions and U(Y)<0 applied incase of negative ions. Voltage offset U(Y) is, in some embodiments, afunction of Y, i.e. the potential of the compensation plates variesalong the drift length, but in this embodiment the voltage offset isconstant. The electrodes 366, 367 are not biased and have zero voltageoffset.

Each ion-optical mirror 310, 311 comprises three elongate mirrorelectrodes. The ion ion-optical mirror 310 comprises three mirrorelectrodes 320, 321, 322 and the ion ion-optical mirror 311 comprisesthree mirror electrodes 330, 331, 332.

The ions in a multi-reflection time-of-flight mass spectrometer 300 arereflected between opposing ion-optical mirrors 310, 311 several timeswhile they are drifting along the direction Y, are reflected and driftback in the direction −Y. It is possible, that the ions are injectedwith a small injection angle Θ to axis X. Accordingly the number of thereflection of the ions and the length of the flight path of the ionswill increase. Due to the multi-refection of the ions at the ion-opticalmirrors 310, 311 comprising several mirror electrodes 320, 321, 322,330, 331, 332, at least one mirror electrode 320, 321, 322, 330, 331,332 of each ion-optical mirrors 310, 311 or each mirror electrode 320,321, 322, 330, 331, 332 of the ion-optical mirrors 310, 311 has to beprovided with an accurate and ultra-stable voltage. Any instabilitycould change the trajectory of the ions which are oscillating betweenthe reflecting ion-optical mirrors 310, 311. Accordingly a changedflight-time of the analysed ions would result in time-of-flight massspectra of lower resolving power or changing mass-to-charge-calibrationof the detected time-of-flight mass spectra.

Therefore, at least inventive voltage supply 2, for example one of thetwelve embodiments described before, is used in the multi-reflectiontime-of-flight mass spectrometer to provide an accurate and ultra-stablereference voltage. The reference voltage is provided to the voltageamplifiers 24 of multi-reflection time-of-flight mass spectrometer,which then provides a voltage to the mirror electrodes 320, 321, 322,330, 331, 332 of ion-optical mirrors 310, 311 of the multi-reflectiontime-of-flight mass analyser 300, which is typically in the range ofkilovolts (kV). Based on the specific concept of the multi-reflectiontime-of-flight mass analyser 230 to each of the mirror electrodes 320,321, 322, 330, 331, 332 of an ion-optical mirrors 310 is the samevoltage provided or more preferably to each of the mirror electrodes320, 321, 322 of an ion-optical mirrors 210 a different voltage isprovided by voltage amplifiers 24 which are using the accurate andultra-stable reference voltage of an inventive voltage supply 2. Onevoltage amplifier 24 can be used to apply the same voltage to more thanone electrode. In particular, typically to electrodes having the samefunction in both ion-optical mirrors 310, 311 the same voltage isapplied by one voltage amplifier 24, for example the outer mirrorelectrodes 320, 330 of both ion-optical mirrors 310, 311. Similarly,another voltage amplifier 24 can supply the same voltage to mirrorelectrodes 321, 331 and a further voltage amplifier 24 can supply thesame voltage to mirror electrodes 322, 332. The voltage applied at themirror electrodes has typically an absolute value in the range of 1 kVup to 12 kV, preferably in the range of 2 KV up to 8 kV. In amulti-reflection time-of-flight mass spectrometer, for example of thisembodiment can be used one inventive voltage supply 2 to provide anaccurate and ultra-stable reference voltage, but also more inventivevoltage supplies 2 to provide accurate and ultra-stable referencevoltages. E.g. to each voltage amplifier 24 can be assigned a separatevoltage supply 2 to provide an accurate and ultra-stable referencevoltage for each voltage amplifier 24.

The embodiments described in this application give examples of theinventive voltage supply and inventive calibration method. So theinvention can be realised by each embodiment alone or by a combinationof several or all features of the described embodiments without anylimitations.

The invention claimed is:
 1. A voltage supply for providing a referencevoltage to supply a voltage to at least one electrode, comprising anultra-stable DC voltage source, an accurate DC voltage source, a tuningunit including a digital to analogue converter, a comparator, and acontrol unit, wherein an ultra-stable voltage is applied to the tuningunit, which is provided based on a supplied voltage of the ultra-stableDC voltage source, the tuning unit provides an output voltage, a voltagebased on the output voltage of the tuning unit is compared by thecomparator with an accurate voltage, which is provided based on asupplied voltage of the accurate DC voltage source and the comparatorprovides a signal resulting from the comparison to the control unit,wherein the control unit is tuning the tuning unit during a tuningperiod according to the signal provided by the comparator to minimisethe absolute difference between the voltage based on the output voltageof the tuning unit and the accurate voltage, and the reference voltageof the voltage supply is provided based on the output voltage of thetuning unit after the tuning period.
 2. The voltage supply according toclaim 1, wherein the supplied voltage of the ultra-stable DC source hasa higher absolute value than the supplied voltage of the accurate DCvoltage source.
 3. The voltage supply according to claim 1, wherein thecontrol unit provides a digital signal to the digital to analogueconverter to tune the output voltage of the digital to analogueconverter.
 4. The voltage supply of claim 3, wherein the digital signalincludes a specific number of bits.
 5. The voltage supply according toclaim 1, wherein the signal provided by the comparator to the controlunit is a digital signal identifying which of the voltages compared bythe comparator has a higher absolute value.
 6. The voltage supplyaccording to claim 5, wherein the control unit provides a signal to thetuning unit according to the digital signal provided by the comparatorto increase the absolute value of the output voltage of the tuning unitif the absolute value of the accurate voltage compared by the comparatoris higher than the absolute value of the voltage based on the outputvoltage of the tuning unit compared by the comparator and to decreasethe absolute value of the output voltage of the tuning unit if theabsolute value of the accurate voltage compared by the comparator islower than the absolute value of the voltage based on output voltage ofthe tuning unit compared by the comparator.
 7. The voltage supplyaccording to claim 5, wherein the control unit provides a digital signalto the digital to analogue converter according to the digital signalprovided by the comparator to increase the absolute value of the outputvoltage of the digital to analogue converter if the absolute value ofthe accurate voltage compared by the comparator is higher than theabsolute value of the voltage based on the output voltage of the digitalto analogue converter compared by the comparator and to decrease theabsolute value of the output voltage of the digital to analogueconverter if the absolute value of the accurate voltage compared by thecomparator is lower than the absolute value of the voltage based onoutput voltage of the digital to analogue converter compared by thecomparator.
 8. The voltage supply according to claim 1, wherein theincrease and decrease of the output voltage of the digital to analogueconverter is reduced stepwise.
 9. The voltage supply according to claim1, wherein the provided reference voltage is applied at a voltageamplifier, which provides an amplified voltage to the at least oneelectrode.
 10. The voltage supply according to claim 9, wherein theprovided reference voltage is applied at the voltage amplifier via aswitch.
 11. The voltage supply according to claim 10, wherein anaccurate voltage, which is provided, based on the supplied voltage ofthe accurate DC voltage source can be applied at the voltage amplifiervia the switch.
 12. The voltage supply according to claim 1, whichprovides the reference voltage to supply a voltage to the electrode of amass spectrometer.
 13. The voltage supply according to claim 12, whereinthe electrode is a center electrode of an electrostatic trap.
 14. Thevoltage supply according to claim 12, wherein the mass spectrometer is atime-of-flight mass spectrometer.
 15. The voltage supply according toclaim 14, wherein the time-of-flight mass spectrometer is amulti-reflection time-of-flight mass spectrometer.
 16. The voltagesupply according to claim 1, wherein the ultra-stable DC voltage sourcehas a voltage stability below 1 ppm over a time period of 24 hours. 17.The voltage supply according to claim 1, wherein the ultra-stable DCvoltage source has a voltage stability below 1 ppm over a temperaturerange of 10° C.
 18. The voltage supply according to claim 1, wherein theaccurate DC voltage source has a production accuracy of its suppliedvoltage below 1000 ppm.
 19. The voltage supply according to claim 1,wherein the supplied voltage of the ultra-stable DC voltage source isthe ultra-stable voltage, which is applied to the tuning unit.
 20. Thevoltage supply according to claim 1, wherein the output voltage of thetuning unit is compared by the comparator with the accurate voltage toprovide the signal to the control unit, wherein the control unit istuning the tuning unit during the tuning period according to the signalto minimise the absolute difference between the output voltage of thetuning unit and the accurate voltage.
 21. The voltage supply accordingto claim 1, wherein the supplied voltage of the accurate DC voltagesource is the accurate voltage, which is compared by the comparator withthe voltage based on the output voltage of the tuning unit.
 22. Thevoltage supply according to claim 1, wherein the reference voltageprovided by the voltage supply is the output voltage of the tuning unitafter the tuning period.
 23. The voltage supply of claim 1, wherein thedigital to analogue converter includes a resistor ladder network.
 24. Amethod for calibrating a voltage supply to provide a reference voltageto supply a voltage to at least one electrode, wherein a voltageprovided based on an output voltage of a tuning unit including a digitalto analogue converter can be provided by the voltage supply via a switchas the reference voltage, comprising the steps activating the voltagesupply, while the voltage provided based on the output voltage of thetuning unit is applied to the switch and this voltage is not providedvia the switch as the reference voltage; tuning the tuning unit by acontrol unit according to the signal provided by a comparator tominimise the absolute difference between the voltage based on the outputvoltage of the tuning unit and the accurate voltage compared by thecomparator; and when the absolute difference between the voltagescompared by the comparator is below a defined minimum value and thetuning by the control unit is stopped, the control unit submits aswitching signal to the switch, the switch receiving the switchingsignal is actuated and then the voltage provided based on the outputvoltage of the tuning unit applied to the switch is provided by thevoltage supply via the switch as the reference voltage.
 25. The methodof claim 24, wherein the voltage provided based on the output voltage ofthe tuning unit applied to the switch is the voltage based on the outputvoltage of the tuning unit, which is compared by the comparator.
 26. Themethod of claim 24, wherein the voltage supply includes the switch, themethod comprising the steps activating the voltage supply, while avoltage provided based on the output voltage of the tuning unit and anaccurate voltage which is provided based on the supplied voltage of theaccurate DC voltage source are applied to the switch and the voltageprovided based on the output voltage of the tuning unit applied to theswitch is not connected via the switch with the voltage amplifier andthe accurate voltage applied to the switch is connected via the switchwith the voltage amplifier; tuning the tuning unit by the control unitaccording to the signal provided by the comparator to minimise theabsolute difference between the voltage based on the output voltage ofthe tuning unit and the accurate voltage compared by the comparator andwhen the absolute difference between the voltages compared by thecomparator is below a defined minimum value and the tuning by thecontrol unit is stopped, the control unit submits a switching signal tothe switch, the switch receiving the switching signal is actuated andthen the voltage provided based on the output voltage of the tuning unitapplied to the switch is connected via the switch with the voltageamplifier as the reference voltage and the accurate voltage, applied tothe switch is disconnected by the switch from the voltage amplifier. 27.The method of claim 26, wherein the voltage provided based on the outputvoltage of the tuning unit applied to the switch is the voltage based onthe output voltage of the tuning unit, which is compared by thecomparator.
 28. The method of claim 26, wherein the accurate voltage,which is applied to the switch, is the accurate voltage, which iscompared by the comparator.